net/netns: support !CAP_NET_ADMIN

netns_linux checked whether "ip rule" could run to determine
whether to use SO_MARK for network namespacing. However in
Linux environments which lack CAP_NET_ADMIN, such as various
container runtimes, the "ip rule" command succeeds but SO_MARK
fails due to lack of permission. SO_BINDTODEVICE would work in
these environments, but isn't tried.

In addition to running "ip rule" check directly whether SO_MARK
works or not. Among others, this allows Microsoft Azure App
Service and AWS App Runner to work.

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

net/netns/netns_linux_test.go

@@ -49,3 +49,9 @@ func TestBypassMarkInSync(t *testing.T) {
 	}
 	t.Errorf("tailscaleBypassMark not found in router_linux.go")
 }
+
+func TestSocketMarkWorks(t *testing.T) {
+	_ = socketMarkWorks()
+	// we cannot actually assert whether the test runner has SO_MARK available
+	// or not, as we don't know. We're just checking that it doesn't panic.
+}