ubifs: Fix unlink code wrt. double hash lookups

When removing an encrypted file with a long name and without having
the key we have to be able to locate and remove the directory entry
via a double hash. This corner case was simply forgotten.

Fixes: 528e3d17 ("ubifs: Add full hash lookup support")
Reported-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>
Signed-off-by: Richard Weinberger <richard@nod.at>