genetlink: switch control commands to per-op policies

In preparation for adding a new attribute to CTRL_CMD_GETPOLICY
split the policies for getpolicy and getfamily apart.

This will cause a slight user-visible change in that dumping
the policies will switch from per family to per op, but
supposedly sniffer-type applications (which are the main use
case for policy dumping thus far) should support both, anyway.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>