IB/srp: Fix a race condition between failing I/O and I/O queueing

Avoid that srp_unmap_data() can get invoked concurrently by
srp_terminate_io() and from the error path in srp_queuecommand()
if srp_post_send() fails.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Cc: Sagi Grimberg <sagig@mellanox.com>
Cc: Sebastian Riemer <sebastian.riemer@profitbricks.com>
Cc: Roland Dreier <roland@kernel.org>