genetlink: allow families to use split ops directly

Let families to hook in the new split ops.

They are more flexible and should not be much larger than
full ops. Each split op is 40B while full op is 48B.
Devlink for example has 54 dos and 19 dumps, 2 of the dumps
do not have a do -> 56 full commands = 2688B.
Split ops would have taken 2920B, so 9% more space while
allowing individual per/post doit and per-type policies.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>