IB/srp: Fix a memory descriptor leak in an error path

If an error occurs after srp_fr_pool_get() succeeded and before the
descriptor is stored in srp_map_state (*state->fr.next++ = desc)
then srp_unmap_data() won't free the newly allocated memory
descriptor. Hence free the descriptor explicitly.

Fixes: f7f7aab1a5c0 ("IB/srp: Convert to new registration API")
Signed-off-by: Bart Van Assche <[e-mail]>
Tested-by: Laurence Oberman <[e-mail]>
Cc: Sagi Grimberg <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Cc: <[e-mail]> # v4.4+
Signed-off-by: Doug Ledford <[e-mail]>

drivers/infiniband/ulp/srp/ib_srp.c

···
 	ib_update_fast_reg_key(desc->mr, rkey);
 
 	n = ib_map_mr_sg(desc->mr, state->sg, sg_nents, 0, dev->mr_page_size);
-	if (unlikely(n < 0))
+	if (unlikely(n < 0)) {
+		srp_fr_pool_put(ch->fr_pool, &desc, 1);
+		pr_debug("%s: ib_map_mr_sg(%d) returned %d.\n",
+			 dev_name(&req->scmnd->device->sdev_gendev), sg_nents,
+			 n);
 		return n;
+	}
 
 	req->reg_cqe.done = srp_reg_mr_err_done;