ubifs: Fix xattr_names length in exit paths

[ Upstream commit 843741c5778398ea67055067f4cc65ae6c80ca0e ]

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

Cc: stable@vger.kernel.org
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>