f2fs: clear append/update flags once fsync is done

When fsync is done through checkpoint, previous f2fs missed to clear append
and update flag. This patch fixes to clear them.

This was originally catched by Changman Lee before.

Signed-off-by: Changman Lee <cm224.lee@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>