genetlink: Add per family bind/unbind callbacks

genetlink: Add per family bind/unbind callbacks

Add genetlink family bind()/unbind() callbacks when adding/removing
multicast group to/from netlink client socket via setsockopt() or
bind() syscall.

They can be used to track if consumers of netlink multicast messages
emerge or disappear. Thus, a client implementing callbacks, can now
send events only when there are active consumers, preventing unnecessary
work when none exist.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240212161615.161935-2-stanislaw.gruszka@linux.intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>