net/dnscache: Handle 4-in-6 addresses in DNS responses

On Android, the system resolver can return IPv4 addresses as IPv6-mapped
addresses (i.e. `::ffff:a.b.c.d`). After the switch to `net/netip`
(19008a3), this case is no longer handled and a response like this will
be seen as failure to resolve any IPv4 addresses.

Handle this case by simply calling `Unmap()` on the returned IP when it
is a 4-in-6 address. Fixes tailscale#5698.

Signed-off-by: Peter Cai <peter@typeblog.net>

net/dnscache/dnscache.go

@@ -278,10 +278,10 @@ func (r *Resolver) lookupIP(host string) (ip, ip6 netip.Addr, allIPs []netip.Add
 
 	have4 := false
 	for _, ipa := range ips {
-		if ipa.Is4() {
+		if ipa.Is4() || ipa.Is4In6() {
 			if !have4 {
 				ip6 = ip
-				ip = ipa
+				ip = ipa.Unmap()
 				have4 = true
 			}
 		} else {