f2fs: Fix wrong stub helper update_sit_info

update_sit_info should be f2fs_update_sit_info,
otherwise build fails while no CONFIG_F2FS_STAT_FS.

Fixes: fc7100ea2a52 ("f2fs: Add f2fs stats to sysfs")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>