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>

cmd/tailscaled/depaware-minbox.txt

@@ -84,7 +84,6 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         gvisor.dev/gvisor/pkg/tcpip/ports                            from gvisor.dev/gvisor/pkg/tcpip/stack+
         gvisor.dev/gvisor/pkg/tcpip/seqnum                           from gvisor.dev/gvisor/pkg/tcpip/header+
      💣 gvisor.dev/gvisor/pkg/tcpip/stack                            from gvisor.dev/gvisor/pkg/tcpip/adapters/gonet+
-        gvisor.dev/gvisor/pkg/tcpip/stack/gro                        from tailscale.com/wgengine/netstack/gro
         gvisor.dev/gvisor/pkg/tcpip/transport                        from gvisor.dev/gvisor/pkg/tcpip/transport/icmp+
         gvisor.dev/gvisor/pkg/tcpip/transport/icmp                   from tailscale.com/wgengine/netstack
         gvisor.dev/gvisor/pkg/tcpip/transport/internal/network       from gvisor.dev/gvisor/pkg/tcpip/transport/icmp+

cmd/tailscaled/deps_test.go

@@ -198,3 +198,14 @@ func TestOmitPortlist(t *testing.T) {
 		},
 	}.Check(t)
 }
+
+func TestOmitGRO(t *testing.T) {
+	deptest.DepChecker{
+		GOOS:   "linux",
+		GOARCH: "amd64",
+		Tags:   "ts_omit_gro,ts_include_cli",
+		BadDeps: map[string]string{
+			"gvisor.dev/gvisor/pkg/tcpip/stack/gro": "unexpected dep with ts_omit_gro",
+		},
+	}.Check(t)
+}