f2fs: fix f2fs_ioc_abort_volatile_write

There are two rules to handle aborting volatile or atomic writes.

1. drop atomic writes
 - we don't need to keep any stale db data.

2. write journal data
 - we should keep the journal data with fsync for db recovery.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>