[NETLINK]: Add netlink_has_listeners for avoiding unneccessary event message generation

Keep a bitmask of multicast groups with subscribed listeners to let
netlink users check for listeners before generating multicast
messages.

Queries don't perform any locking, which may result in false
positives, it is guaranteed however that any new subscriptions are
visible before bind() or setsockopt() return.

Signed-off-by: Patrick McHardy <kaber@trash.net>
ACKed-by: Jamal Hadi <Salim&lt;hadi@cyberus.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>