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>

drivers/infiniband/ulp/srp/ib_srp.c

@@ -1594,6 +1594,12 @@ err_unmap:
 err_iu:
 	srp_put_tx_iu(target, iu, SRP_IU_CMD);
 
+	/*
+	 * Avoid that the loops that iterate over the request ring can
+	 * encounter a dangling SCSI command pointer.
+	 */
+	req->scmnd = NULL;
+
 	spin_lock_irqsave(&target->lock, flags);
 	list_add(&req->list, &target->free_reqs);