cmd/containerboot,util/linuxfw: create a SNAT rule for dst/src only once, clean up if needed (#13658)

The AddSNATRuleForDst rule was adding a new rule each time it was called including:
- if a rule already existed
- if a rule matching the destination, but with different desired source already existed

This was causing issues especially for the in-progress egress HA proxies work,
where the rules are now refreshed more frequently, so more redundant rules
were being created.

This change:
- only creates the rule if it doesn't already exist
- if a rule for the same dst, but different source is found, delete it
- also ensures that egress proxies refresh firewall rules
if the node's tailnet IP changes

Updates tailscale/tailscale#13406

Signed-off-by: Irbe Krumina <irbe@tailscale.com>

cmd/containerboot/forwarding.go

@@ -117,7 +117,7 @@ func installEgressForwardingRule(_ context.Context, dstStr string, tsIPs []netip
 	if err := nfr.DNATNonTailscaleTraffic("tailscale0", dst); err != nil {
 		return fmt.Errorf("installing egress proxy rules: %w", err)
 	}
-	if err := nfr.AddSNATRuleForDst(local, dst); err != nil {
+	if err := nfr.EnsureSNATForDst(local, dst); err != nil {
 		return fmt.Errorf("installing egress proxy rules: %w", err)
 	}
 	if err := nfr.ClampMSSToPMTU("tailscale0", dst); err != nil {