audit: send multicast messages only if there are listeners

Test first to see if there are any userspace multicast listeners bound to the
socket before starting the multicast send work.

Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/audit.c

@@ -435,6 +435,9 @@ static void kauditd_send_multicast_skb(struct sk_buff *skb)
 	struct audit_net	*aunet = net_generic(&init_net, audit_net_id);
 	struct sock		*sock = aunet->nlsk;
 
+	if (!netlink_has_listeners(sock, AUDIT_NLGRP_READLOG))
+		return;
+
 	/*
 	 * The seemingly wasteful skb_copy() rather than bumping the refcount
 	 * using skb_get() is necessary because non-standard mods are made to