Use "hash" value in rtalloc_mpath_fib() instead of RTF_ANNOUNCE flag.

Hashing method is the same as in in6_src.c. (Probably we need better one).

MFC after:	2 weeks

sys/netinet6/nd6_nbr.c

@@ -242,7 +242,7 @@ nd6_ns_input(struct mbuf *m, int off, int icmp6len)
 
 		/* Always use the default FIB. */
 #ifdef RADIX_MPATH
-		rtalloc_mpath_fib((struct route *)&ro, RTF_ANNOUNCE,
+		rtalloc_mpath_fib((struct route *)&ro, ntohl(taddr6.s6_addr32[3]),
 		    RT_DEFAULT_FIB);
 #else
 		in6_rtalloc(&ro, RT_DEFAULT_FIB);