[NET]: Fix IP_ADD/DROP_MEMBERSHIP to handle only connectionless

Fix IP[V6]_ADD_MEMBERSHIP and IP[V6]_DROP_MEMBERSHIP to
return -EPROTO for connection oriented sockets.

Signed-off-by: Flavio Leitner <fleitner@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>