RDMA/rxe: Do not NULL deref on debugging failure path

Correct the mistake, mr is obviously NULL in this code path.

Fixes: 2778b72b1d ("RDMA/rxe: Replace pr_xxx by rxe_dbg_xxx in rxe_mr.c")
Link: https://lore.kernel.org/r/Y3eeJW0AdyJYhYyQ@kili
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/sw/rxe/rxe_mr.c

@@ -519,7 +519,7 @@ int rxe_invalidate_mr(struct rxe_qp *qp, u32 key)
 
 	mr = rxe_pool_get_index(&rxe->mr_pool, key >> 8);
 	if (!mr) {
-		rxe_dbg_mr(mr, "No MR for key %#x\n", key);
+		rxe_dbg_qp(qp, "No MR for key %#x\n", key);
 		ret = -EINVAL;
 		goto err;
 	}