RDMA/mlx5: Return proper error value

Returned value from mlx5_mr_cache_alloc() is checked to be error or real
pointer. Return proper error code instead of NULL which is not checked
later.

Fixes: 81713d37 ("IB/mlx5: Add implicit MR support")
Link: https://lore.kernel.org/r/20191029055721.7192-1-leon@kernel.orgSigned-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/hw/mlx5/mr.c

@@ -411,7 +411,7 @@ struct mlx5_ib_mr *mlx5_mr_cache_alloc(struct mlx5_ib_dev *dev, int entry)
 
 	if (entry < 0 || entry >= MAX_MR_CACHE_ENTRIES) {
 		mlx5_ib_err(dev, "cache entry %d is out of range\n", entry);
-		return NULL;
+		return ERR_PTR(-EINVAL);
 	}
 
 	ent = &cache->ent[entry];