ipn: when enforcing defaults, set UsePacketFilter=true

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

ipn/prefs.go

@@ -156,6 +156,7 @@ func PrefsFromBytes(b []byte, enforceDefaults bool) (*Prefs, error) {
 	if enforceDefaults {
 		p.RouteAll = true
 		p.AllowSingleHosts = true
+		p.UsePacketFilter = true
 	}
 	return p, err
 }