ubifs: Fix xattr generic handler usage

UBIFS uses full names to work with xattrs, therefore we have to use
xattr_full_name() to obtain the xattr prefix as string.

Cc: <stable@vger.kernel.org>
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Fixes: 2b88fc21

 ("ubifs: Switch to generic xattr handlers")
Signed-off-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Andreas Gruenbacher <agruenba@redhat.com>
Tested-by: Dongsheng Yang <dongsheng081251@gmail.com>