RDMA/mlx5: Protect from NULL pointer derefence

The mlx5_ib_alloc_implicit_mr() can fail to acquire pages
and the returned mr pointer won't be valid. Ensure that it
is not error prior to access.

Cc: <stable@vger.kernel.org> # 4.10
Fixes: 81713d37 ("IB/mlx5: Add implicit MR support")
Reported-by: Noa Osherovich <noaos@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/hw/mlx5/mr.c

@@ -1232,6 +1232,8 @@ struct ib_mr *mlx5_ib_reg_user_mr(struct ib_pd *pd, u64 start, u64 length,
 			return ERR_PTR(-EINVAL);
 
 		mr = mlx5_ib_alloc_implicit_mr(to_mpd(pd), access_flags);
+		if (IS_ERR(mr))
+			return ERR_CAST(mr);
 		return &mr->ibmr;
 	}
 #endif