Fix a crash with multipath activated. It happends when ANA log
page is larger than MDTS and because of that ANA is disabled.
The driver then tries to access unallocated buffer when connecting
to a nvme target. The signature is as follows:

[  300.433586] nvme nvme0: ANA log page size (8208) larger than MDTS (8192).
[  300.435387] nvme nvme0: disabling ANA support.
[  300.437835] nvme nvme0: creating 4 I/O queues.
[  300.459132] nvme nvme0: new ctrl: NQN "nqn.0.0.0", addr 10.91.0.1:8009
[  300.464609] BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
[  300.466342] #PF error: [normal kernel read fault]
[  300.467385] PGD 0 P4D 0
[  300.467987] Oops: 0000 [#1] SMP PTI
[  300.468787] CPU: 3 PID: 50 Comm: kworker/u8:1 Not tainted 5.0.20kalray+ #4
[  300.470264] Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
[  300.471532] Workqueue: nvme-wq nvme_scan_work [nvme_core]
[  300.472724] RIP: 0010:nvme_parse_ana_log+0x21/0x140 [nvme_core]
[  300.474038] Code: 45 01 d2 d8 48 98 c3 66 90 0f 1f 44 00 00 41 57 41 56 41 55 41 54 55 53 48 89 fb 48 83 ec 08 48 8b af 20 0a 00 00 48 89 34 24 <66> 83 7d 08 00 0f 84 c6 00 00 00 44 8b 7d 14 49 89 d5 8b 55 10 48
[  300.477374] RSP: 0018:ffffa50e80fd7cb8 EFLAGS: 00010296
[  300.478334] RAX: 0000000000000001 RBX: ffff9130f1872258 RCX: 0000000000000000
[  300.479784] RDX: ffffffffc06c4c30 RSI: ffff9130edad4280 RDI: ffff9130f1872258
[  300.481488] RBP: 0000000000000000 R08: 0000000000000001 R09: 0000000000000044
[  300.483203] R10: 0000000000000220 R11: 0000000000000040 R12: ffff9130f18722c0
[  300.484928] R13: ffff9130f18722d0 R14: ffff9130edad4280 R15: ffff9130f18722c0
[  300.486626] FS:  0000000000000000(0000) GS:ffff9130f7b80000(0000) knlGS:0000000000000000
[  300.488538] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  300.489907] CR2: 0000000000000008 CR3: 00000002365e6000 CR4: 00000000000006e0
[  300.491612] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  300.493303] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  300.494991] Call Trace:
[  300.495645]  nvme_mpath_add_disk+0x5c/0xb0 [nvme_core]
[  300.496880]  nvme_validate_ns+0x2ef/0x550 [nvme_core]
[  300.498105]  ? nvme_identify_ctrl.isra.45+0x6a/0xb0 [nvme_core]
[  300.499539]  nvme_scan_work+0x2b4/0x370 [nvme_core]
[  300.500717]  ? __switch_to_asm+0x35/0x70
[  300.501663]  process_one_work+0x171/0x380
[  300.502340]  worker_thread+0x49/0x3f0
[  300.503079]  kthread+0xf8/0x130
[  300.503795]  ? max_active_store+0x80/0x80
[  300.504690]  ? kthread_bind+0x10/0x10
[  300.505502]  ret_from_fork+0x35/0x40
[  300.506280] Modules linked in: nvme_tcp nvme_rdma rdma_cm iw_cm ib_cm ib_core nvme_fabrics nvme_core xt_physdev ip6table_raw ip6table_mangle ip6table_filter ip6_tables xt_comment iptable_nat nf_nat_ipv4 nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 xt_CHECKSUM iptable_mangle iptable_filter veth ebtable_filter ebtable_nat ebtables iptable_raw vxlan ip6_udp_tunnel udp_tunnel sunrpc joydev pcspkr virtio_balloon br_netfilter bridge stp llc ip_tables xfs libcrc32c ata_generic pata_acpi virtio_net virtio_console net_failover virtio_blk failover ata_piix serio_raw libata virtio_pci virtio_ring virtio
[  300.514984] CR2: 0000000000000008
[  300.515569] ---[ end trace faa2eefad7e7f218 ]---
[  300.516354] RIP: 0010:nvme_parse_ana_log+0x21/0x140 [nvme_core]
[  300.517330] Code: 45 01 d2 d8 48 98 c3 66 90 0f 1f 44 00 00 41 57 41 56 41 55 41 54 55 53 48 89 fb 48 83 ec 08 48 8b af 20 0a 00 00 48 89 34 24 <66> 83 7d 08 00 0f 84 c6 00 00 00 44 8b 7d 14 49 89 d5 8b 55 10 48
[  300.520353] RSP: 0018:ffffa50e80fd7cb8 EFLAGS: 00010296
[  300.521229] RAX: 0000000000000001 RBX: ffff9130f1872258 RCX: 0000000000000000
[  300.522399] RDX: ffffffffc06c4c30 RSI: ffff9130edad4280 RDI: ffff9130f1872258
[  300.523560] RBP: 0000000000000000 R08: 0000000000000001 R09: 0000000000000044
[  300.524734] R10: 0000000000000220 R11: 0000000000000040 R12: ffff9130f18722c0
[  300.525915] R13: ffff9130f18722d0 R14: ffff9130edad4280 R15: ffff9130f18722c0
[  300.527084] FS:  0000000000000000(0000) GS:ffff9130f7b80000(0000) knlGS:0000000000000000
[  300.528396] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  300.529440] CR2: 0000000000000008 CR3: 00000002365e6000 CR4: 00000000000006e0
[  300.530739] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  300.531989] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  300.533264] Kernel panic - not syncing: Fatal exception
[  300.534338] Kernel Offset: 0x17c00000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
[  300.536227] ---[ end Kernel panic - not syncing: Fatal exception ]---

Condition check refactoring from Christoph Hellwig.

Signed-off-by: Marta Rybczynska <marta.rybczynska@kalray.eu>
Tested-by: Jean-Baptiste Riaux <jbriaux@kalray.eu>
Signed-off-by: Christoph Hellwig <hch@lst.de>