util/linuxfw: add missing input rule to the tailscale tun

Add an explicit accept rule for input to the tun interface, as a mirror
to the explicit rule to accept output from the tun interface.

The rule matches any packet in to our tun interface and accepts it, and
the rule is positioned and prioritized such that it should be evaluated
prior to conventional ufw/iptables/nft rules.

Updates #391
Fixes #7332
Updates #9084

Signed-off-by: James Tucker <james@tailscale.com>

util/linuxfw/iptables_runner_test.go

@@ -261,6 +261,7 @@ func TestAddAndDeleteBase(t *testing.T) {
 	}
 
 	tsRulesCommon := []fakeRule{ // table/chain/rule
+		{"filter", "ts-input", []string{"-i", tunname, "-j", "ACCEPT"}},
 		{"filter", "ts-forward", []string{"-i", tunname, "-j", "MARK", "--set-mark", TailscaleSubnetRouteMark + "/" + TailscaleFwmarkMask}},
 		{"filter", "ts-forward", []string{"-m", "mark", "--mark", TailscaleSubnetRouteMark + "/" + TailscaleFwmarkMask, "-j", "ACCEPT"}},
 		{"filter", "ts-forward", []string{"-o", tunname, "-j", "ACCEPT"}},