ubifs: orphan: Handle xattrs like files

Like for the journal case, make sure that we track all xattr
inodes.
Otherwise UBIFS might not be able to locate stale xattr inodes
upon recovery.

Reported-by: Stefan Agner <stefan@agner.ch>
Fixes: 1e51764a ("UBIFS: add new flash file system")
Signed-off-by: Richard Weinberger <richard@nod.at>