Since 6.8-rc1, using VTABLE_DEFAULT_ERROR for optional functions
(never called) in #[vtable] is the recommended way.

Note that no functional changes in this patch.
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>