ubifs: Code cleanup by removing ifdef macro surrounding

Define ubifs_listxattr and ubifs_xattr_handlers to NULL
when CONFIG_UBIFS_FS_XATTR is not enabled, then we can
remove many ugly ifdef macros in the code.

Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
Signed-off-by: Richard Weinberger <richard@nod.at>