IP multicast code no longer needs to acquire Giant before appending

an mbuf onto a socket buffer.  This is left over from debug.mpsafenet
affecting the forwarding/bridging plane only.

sys/netinet/ip_mroute.c

@@ -1304,13 +1304,10 @@ static int
 socket_send(struct socket *s, struct mbuf *mm, struct sockaddr_in *src)
 {
     if (s) {
-	mtx_lock(&Giant);		/* XXX until sockets are locked */
 	if (sbappendaddr(&s->so_rcv, (struct sockaddr *)src, mm, NULL) != 0) {
 	    sorwakeup(s);
-	    mtx_unlock(&Giant);
 	    return 0;
 	}
-	mtx_unlock(&Giant);
     }
     m_freem(mm);
     return -1;