ndp: Don't sabotage DAD by replying to probing neighbour solicitation

If the solicitation comes from ::, it's the guest performing
duplicate address detection -- don't answer that.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

ndp.c

@@ -76,6 +76,9 @@ int ndp(struct ctx *c, struct ethhdr *eh, size_t len)
 			  sizeof(struct in6_addr))
 			return -1;
 
+		if (IN6_IS_ADDR_UNSPECIFIED(&ip6h->saddr))
+			return 1;
+
 		info("NDP: received NS, sending NA");
 		ihr->icmp6_type = NA;
 		ihr->icmp6_code = 0;