f2fs: move f2fs_balance_fs to punch_hole

The f2fs_fallocate() has two operations: punch_hole and expand_size.

Only in the case of punch_hole, dirty node pages can be produced, so let's
trigger f2fs_balance_fs() in this case only.
Furthermore, let's trigger it at every data truncation routine.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>