util/linuxfw: fix stateful packet filtering in nftables mode

To match iptables:
b5dbf155b1/util/linuxfw/iptables_runner.go (L536)

Updates #12066

Signed-off-by: Anton Tolchanov <anton@tailscale.com>
(cherry picked from commit ac638f32c0)

util/linuxfw/nftables_runner.go

@@ -1773,7 +1773,7 @@ func makeStatefulRuleExprs(tunname string) []expr.Any {
 		// going to our TUN.
 		&expr.Meta{Key: expr.MetaKeyOIFNAME, Register: 1},
 		&expr.Cmp{
-			Op:       expr.CmpOpNeq,
+			Op:       expr.CmpOpEq,
 			Register: 1,
 			Data:     []byte(tunname),
 		},