UBIFS: do not write orphans back

Orphan inodes are deleted inodes which will disappear after FS
re-mount. There is not need to write orphan inodes back, because
they are not needed on the flash media.

So optimize orphans a little by not writing them back. Just mark
them as clean, free the budget, and report success to VFS.

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