net/mlx5: Fix dereference on pointer attr after null check

When removing a flow from the slow path fdb, a flow attr struct is
allocated for the rule removal process. If the allocation fails the
code prints a warning message but continues with the removal flow
which include dereferencing a pointer which could be null.
Fix this by exiting the function in case the attr allocation failed.

Fixes: c620b772 ("net/mlx5: Refactor tc flow attributes structure")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ariel Levkovich <lariel@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>

drivers/net/ethernet/mellanox/mlx5/core/en_tc.c

@@ -1238,8 +1238,10 @@ mlx5e_tc_unoffload_from_slow_path(struct mlx5_eswitch *esw,
 	struct mlx5_flow_attr *slow_attr;
 
 	slow_attr = mlx5_alloc_flow_attr(MLX5_FLOW_NAMESPACE_FDB);
-	if (!slow_attr)
-		mlx5_core_warn(flow->priv->mdev, "Unable to unoffload slow path rule\n");
+	if (!slow_attr) {
+		mlx5_core_warn(flow->priv->mdev, "Unable to alloc attr to unoffload slow path rule\n");
+		return;
+	}
 
 	memcpy(slow_attr, flow->attr, ESW_FLOW_ATTR_SZ);
 	slow_attr->action = MLX5_FLOW_CONTEXT_ACTION_FWD_DEST;