net/mlx5: handle errors in mlx5_chains_create_table()

In the Linux kernel, the following vulnerability has been resolved: net/mlx5: handle errors in mlx5_chains_create_table() In mlx5_chains_create_table(), the return value of mlx5_get_fdb_sub_ns() and mlx5_get_flow_namespace() must be checked to prevent NULL pointer dereferences. If either function fails, the function should log error message with mlx5_core_warn() and return error pointer.