Count multicast packets received for groups of which we are not

a member separately from generic ``can't forward'' packets.  This
would have helped me find the previous bug much faster.

sys/netinet/ip_input.c

@@ -412,7 +412,7 @@ tooshort:
 		 */
 		IN_LOOKUP_MULTI(ip->ip_dst, m->m_pkthdr.rcvif, inm);
 		if (inm == NULL) {
-			ipstat.ips_cantforward++;
+			ipstat.ips_notmember++;
 			m_freem(m);
 			return;
 		}

sys/netinet/ip_var.h

@@ -147,6 +147,7 @@ struct	ipstat {
 	u_long	ips_badvers;		/* ip version != 4 */
 	u_long	ips_rawout;		/* total raw ip packets generated */
 	u_long	ips_toolong;		/* ip length > max ip packet size */
+	u_long	ips_notmember;		/* multicasts for unregistered grps */
 };
 
 #ifdef KERNEL