wgengine/netstack/gro: permit building without GRO

This only saves ~32KB in the minimal linux/amd64 binary, but it's a
step towards permitting not depending on gvisor for small builds.

Updates #17283

Change-Id: Iae8da5e9465127de354dbcaf25e794a6832d891b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/netstack/gro/gro_disabled.go

@@ -0,0 +1,28 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build ios || ts_omit_gro
+
+package gro
+
+import (
+	"runtime"
+
+	"tailscale.com/net/packet"
+)
+
+type GRO struct{}
+
+func NewGRO() *GRO {
+	if runtime.GOOS == "ios" {
+		panic("unsupported on iOS")
+	}
+	panic("GRO disabled in build")
+
+}
+
+func (g *GRO) SetDispatcher(any) {}
+
+func (g *GRO) Enqueue(_ *packet.Parsed) {}
+
+func (g *GRO) Flush() {}