IB/srp: Fix a sporadic crash triggered by cable pulling

Avoid that the loops that iterate over the request ring can encounter
a pointer to a SCSI command in req->scmnd that is no longer associated
with that request. If the function srp_unmap_data() is invoked twice
for a SCSI command that is not in flight then that would cause
ib_fmr_pool_unmap() to be invoked with an invalid pointer as argument,
resulting in a kernel oops.

Reported-by: Sagi Grimberg <sagig@mellanox.com>
Reference: http://thread.gmane.org/gmane.linux.drivers.rdma/19068/focus=19069


Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>