UBIFS: fix file-system synchronization

Argh. The ->sync_fs call is called _before_ all inodes are flushed.
This means we first sync write buffers and commit, then all
inodes are synced, and we end up with unflushed write buffers!

Fix this by forcing synching all indoes from 'ubifs_sync_fs()'.

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