ubifs: Check link count of inodes when killing orphans.

O_TMPFILE files can change their link count back to non-zero.
This corner case needs to get addressed in the orphans subsystem
too.

Fixes: 474b9370 ("ubifs: Implement O_TMPFILE")
Reported-by: Lars Persson <lists@bofh.nu>
Signed-off-by: Richard Weinberger <richard@nod.at>