ubifs: Fix xattr_names length in exit paths

commit 843741c5778398ea67055067f4cc65ae6c80ca0e 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: Willy Tarreau <w@1wt.eu>