f2fs: remove useless parameter of __insert_free_nid()

In current version, @state will only be FREE_NID. This parameter
has no real effect so remove it to keep clean.

Signed-off-by: Liu Song <liu.song11@zte.com.cn>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>