net/batching: fix compile-time assert (#16864)

Updates #cleanup

Signed-off-by: Jordan Whited <jordan@tailscale.com>

net/batching/conn_linux.go

@@ -45,7 +45,7 @@ type xnetBatchWriter interface {
 
 var (
 	// [linuxBatchingConn] implements [Conn].
-	_ Conn = &linuxBatchingConn{}
+	_ Conn = (*linuxBatchingConn)(nil)
 )
 
 // linuxBatchingConn is a UDP socket that provides batched i/o. It implements