f2fs: do f2fs_balance_fs in front of dir operations

In order to conserve free sections to deal with the worst-case scenarios, f2fs
should be able to freeze all the directory operations especially when there are
not enough free sections. The f2fs_balance_fs() is for this use.

When FS utilization becomes almost 100%, directory operations can be failed due
to -ENOSPC frequently, which produces some dirty node pages occasionally.

Previously, in such a case, f2fs_balance_fs() is not able to be triggered since
it is triggered only if the directory operation ends up with success.

So, this patch triggers f2fs_balance_fs() at first before handling directory
operations.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>