Make the code for mapping an iovec more robust by checking the bounds of
the allocated iovec. This patch avoids that the following crash occurs if a
map attempt is made that exceeds the bounds of the iovec that is being
mapped:

BUG: unable to handle kernel NULL pointer dereference at 00000000
00000014
RIP: 0010:iscsit_map_iovec+0x120/0x190 [iscsi_target_mod]
Call Trace:
 iscsit_get_rx_pdu+0x8a2/0xe00 [iscsi_target_mod]
 iscsi_target_rx_thread+0x6e/0xa0 [iscsi_target_mod]
 kthread+0x109/0x140

Cc: Mike Christie <mchristi@redhat.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>