ubifs: Fix xattr_names length in exit paths

commit 843741c5 upstream.

When the operation fails we also have to undo the changes
we made to ->xattr_names. Otherwise listxattr() will report
wrong lengths.

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>