wgengine/netstack: use build tags to exclude gVisor GRO importation on iOS (#13015)

Updates tailscale/corp#22125

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

wgengine/netstack/gro_ios.go

@@ -0,0 +1,30 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build ios
+
+package netstack
+
+import (
+	"gvisor.dev/gvisor/pkg/tcpip/stack"
+)
+
+// gro on iOS delivers packets to its Dispatcher, immediately. This type exists
+// to prevent importation of the gVisor GRO implementation as said package
+// increases binary size. This is a penalty we do not wish to pay since we
+// currently do not leverage GRO on iOS.
+type gro struct {
+	Dispatcher stack.NetworkDispatcher
+}
+
+func (g *gro) Init(v bool) {
+	if v {
+		panic("GRO is not supported on this platform")
+	}
+}
+
+func (g *gro) Flush() {}
+
+func (g *gro) Enqueue(pkt *stack.PacketBuffer) {
+	g.Dispatcher.DeliverNetworkPacket(pkt.NetworkProtocolNumber, pkt)
+}