UBIFS: always commit in sync_fs

Always run commit in sync_fs, because even if the journal seems
to be almost empty, there may be a deletion which removes a large
file, which affects the index greatly. And because we want
better free space predictions after 'sync_fs()', we have to
commit.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>