Commit 1fa5efe3 (ext4: Use generic helpers for quotaon
and quotaoff) made possible to call quotactl(Q_XQUOTAON/OFF) on ext4 filesystems
with sysfile quota support. This leads to calling dquot_enable/disable without s_umount
held in excl. mode, because quotactl_cmd_onoff checks only for Q_QUOTAON/OFF.

The following WARN_ON_ONCE triggers (in this case for dquot_enable, ext4, latest Linus' tree):

[  117.807056] EXT4-fs (dm-0): mounted filesystem with ordered data mode. Opts: quota,prjquota

[...]

[  155.036847] WARNING: CPU: 0 PID: 2343 at fs/quota/dquot.c:2469 dquot_enable+0x34/0xb9
[  155.036851] Modules linked in: quota_v2 quota_tree ipv6 af_packet joydev mousedev psmouse serio_raw pcspkr i2c_piix4 intel_agp intel_gtt e1000 ttm drm_kms_helper drm agpgart fb_sys_fops syscopyarea sysfillrect sysimgblt i2c_core input_leds kvm_intel kvm irqbypass qemu_fw_cfg floppy evdev parport_pc parport button crc32c_generic dm_mod ata_generic pata_acpi ata_piix libata loop ext4 crc16 mbcache jbd2 usb_storage usbcore sd_mod scsi_mod
[  155.036901] CPU: 0 PID: 2343 Comm: qctl Not tainted 4.20.0-rc6-00025-gf5d58277

 #9
[  155.036903] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
[  155.036911] RIP: 0010:dquot_enable+0x34/0xb9
[  155.036915] Code: 41 56 41 55 41 54 55 53 4c 8b 6f 28 74 02 0f 0b 4d 8d 7d 70 49 89 fc 89 cb 41 89 d6 89 f5 4c 89 ff e8 23 09 ea ff 85 c0 74 0a <0f> 0b 4c 89 ff e8 8b 09 ea ff 85 db 74 6a 41 8b b5 f8 00 00 00 0f
[  155.036918] RSP: 0018:ffffb09b00493e08 EFLAGS: 00010202
[  155.036922] RAX: 0000000000000001 RBX: 0000000000000008 RCX: 0000000000000008
[  155.036924] RDX: 0000000000000001 RSI: 0000000000000002 RDI: ffff9781b67cd870
[  155.036926] RBP: 0000000000000002 R08: 0000000000000000 R09: 61c8864680b583eb
[  155.036929] R10: ffffb09b00493e48 R11: ffffffffff7ce7d4 R12: ffff9781b7ee8d78
[  155.036932] R13: ffff9781b67cd800 R14: 0000000000000004 R15: ffff9781b67cd870
[  155.036936] FS:  00007fd813250b88(0000) GS:ffff9781ba000000(0000) knlGS:0000000000000000
[  155.036939] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  155.036942] CR2: 00007fd812ff61d6 CR3: 000000007c882000 CR4: 00000000000006b0
[  155.036951] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  155.036953] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  155.036955] Call Trace:
[  155.037004]  dquot_quota_enable+0x8b/0xd0
[  155.037011]  kernel_quotactl+0x628/0x74e
[  155.037027]  ? do_mprotect_pkey+0x2a6/0x2cd
[  155.037034]  __x64_sys_quotactl+0x1a/0x1d
[  155.037041]  do_syscall_64+0x55/0xe4
[  155.037078]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  155.037105] RIP: 0033:0x7fd812fe1198
[  155.037109] Code: 02 77 0d 48 89 c1 48 c1 e9 3f 75 04 48 8b 04 24 48 83 c4 50 5b c3 48 83 ec 08 49 89 ca 48 63 d2 48 63 ff b8 b3 00 00 00 0f 05 <48> 89 c7 e8 c1 eb ff ff 5a c3 48 63 ff b8 bb 00 00 00 0f 05 48 89
[  155.037112] RSP: 002b:00007ffe8cd7b050 EFLAGS: 00000206 ORIG_RAX: 00000000000000b3
[  155.037116] RAX: ffffffffffffffda RBX: 00007ffe8cd7b148 RCX: 00007fd812fe1198
[  155.037119] RDX: 0000000000000000 RSI: 00007ffe8cd7cea9 RDI: 0000000000580102
[  155.037121] RBP: 00007ffe8cd7b0f0 R08: 000055fc8eba8a9d R09: 0000000000000000
[  155.037124] R10: 00007ffe8cd7b074 R11: 0000000000000206 R12: 00007ffe8cd7b168
[  155.037126] R13: 000055fc8eba8897 R14: 0000000000000000 R15: 0000000000000000
[  155.037131] ---[ end trace 210f864257175c51 ]---

and then the syscall proceeds without s_umount locking.

This patch locks the superblock ->s_umount sem. in exclusive mode for all Q_XQUOTAON/OFF
quotactls too in addition to Q_QUOTAON/OFF.

AFAICT, other than ext4, only xfs and ocfs2 are affected by this change.
The VFS will now call in xfs_quota_* functions with s_umount held, which wasn't the case
before. This looks good to me but I can not say for sure. Ext4 and ocfs2 where already
beeing called with s_umount exclusive via quota_quotaon/off which is basically the same.

Signed-off-by: Javier Barrio <javier.barrio.mart@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>