- fixed a bug that we can't send a packet to ipv4mapped ipv6 address

using a udp6 socket without bind(2)ing.
- fbsd4/430 reported from the FreeBSD team.
- this fix is different from the fix reported in the above PR.  i think
  this better, but we need some test.

Obtained from:	KAME
MFC after:	3 weeks

sys/netinet6/udp6_usrreq.c

@@ -538,6 +538,8 @@ udp6_attach(struct socket *so, int proto, struct thread *td)
 		return error;
 	inp = (struct inpcb *)so->so_pcb;
 	inp->inp_vflag |= INP_IPV6;
+	if (ip6_mapped_addr_on)
+		inp->inp_vflag |= INP_IPV4;
 	inp->in6p_hops = -1;	/* use kernel default */
 	inp->in6p_cksum = -1;	/* just to be sure */
 	/*