genetlink: add iterator for walking family ops

Subsequent changes will expose split op structures to users,
so walking the family ops with just an index will get harder.
Add a structured iterator, convert the simple cases.
Policy dumping needs more careful conversion.

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>