scsi_transport_srp: Fix a race condition

Avoid that srp_terminate_io() can get invoked while srp_queuecommand()
is in progress. This patch avoids that an I/O timeout can trigger the
following kernel warning:

WARNING: at drivers/infiniband/ulp/srp/ib_srp.c:1447 srp_terminate_io+0xef/0x100 [ib_srp]()
Call Trace:
 [<ffffffff814c65a2>] dump_stack+0x4e/0x68
 [<ffffffff81051f71>] warn_slowpath_common+0x81/0xa0
 [<ffffffff8105204a>] warn_slowpath_null+0x1a/0x20
 [<ffffffffa075f51f>] srp_terminate_io+0xef/0x100 [ib_srp]
 [<ffffffffa07495da>] __rport_fail_io_fast+0xba/0xc0 [scsi_transport_srp]
 [<ffffffffa0749a90>] rport_fast_io_fail_timedout+0xe0/0xf0 [scsi_transport_srp]
 [<ffffffff8106e09b>] process_one_work+0x1db/0x780
 [<ffffffff8106e75b>] worker_thread+0x11b/0x450
 [<ffffffff81073c64>] kthread+0xe4/0x100
 [<ffffffff814cf26c>] ret_from_fork+0x7c/0xb0

See also patch "scsi_transport_srp: Add transport layer error
handling" (commit ID 29c17324).

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: James Bottomley <JBottomley@Odin.com>
Cc: Sagi Grimberg <sagig@mellanox.com>
Cc: Sebastian Parschauer <sebastian.riemer@profitbricks.com>
Cc: <stable@vger.kernel.org> #v3.13
Signed-off-by: Doug Ledford <dledford@redhat.com>