genetlink: convert control family to split ops

Prove that the split ops work.
Sadly we need to keep bug-wards compatibility and specify
the same policy for dump as do, even tho we don't parse
inputs for the dump.

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>