wgengine: don't lose filter state on filter reconfig.

We were abandoning the UDP port LRU every time we got a new packet
filter from tailcontrol, which caused return packets to suddenly stop
arriving.

ipn/local.go

@@ -320,7 +320,7 @@ func (b *LocalBackend) updateFilter(netMap *controlclient.NetworkMap) {
 		} else {
 			b.logf("netmap packet filter: (suppressed)\n")
 		}
-		b.e.SetFilter(filter.New(netMap.PacketFilter))
+		b.e.SetFilter(filter.New(netMap.PacketFilter, b.e.GetFilter()))
 	}
 }