net/dnsfallback: more explicitly pass through logf function

Redoes the approach from #5550 and #7539 to explicitly pass in the logf
function, instead of having global state that can be overridden.

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

control/controlclient/direct.go

@@ -211,7 +211,7 @@ func NewDirect(opts Options) (*Direct, error) {
 		dnsCache := &dnscache.Resolver{
 			Forward:          dnscache.Get().Forward, // use default cache's forwarder
 			UseLastGood:      true,
-			LookupIPFallback: dnsfallback.Lookup,
+			LookupIPFallback: dnsfallback.Lookup(opts.Logf),
 			Logf:             opts.Logf,
 		}
 		tr := http.DefaultTransport.(*http.Transport).Clone()