IB/mlx4: Set umem field to NULL in mlx4_ib_alloc_fast_reg_mr()

Set mr->umem to NULL in mlx4_ib_alloc_fast_reg_mr(). Otherwise
ib_dereg_mr() may invoke ib_umem_release() on a random pointer value
and get an oops.
Signed-off-by: Vladimir Sokolovsky <vlad@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/hw/mlx4/mr.c

@@ -205,6 +205,7 @@ struct ib_mr *mlx4_ib_alloc_fast_reg_mr(struct ib_pd *pd,
 		goto err_mr;
 
 	mr->ibmr.rkey = mr->ibmr.lkey = mr->mmr.key;
+	mr->umem = NULL;
 
 	return &mr->ibmr;