f2fs: mark is_extension_exist() inline

The caller set_file_temperature() is marked as inline as well.
It doesn't make much sense to leave is_extension_exist() un-inlined.

Signed-off-by: Park Ju Hyung <qkrwngud825@gmail.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>