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/services.go

@@ -196,8 +196,7 @@ func (ep *egressProxy) syncEgressConfigs(cfgs *egressservices.Configs, status *e
 				if !local.IsValid() {
 					return nil, fmt.Errorf("no valid local IP: %v", local)
 				}
-				// TODO(irbekrm): only create the SNAT rule if it does not already exist.
-				if err := ep.nfr.AddSNATRuleForDst(local, t); err != nil {
+				if err := ep.nfr.EnsureSNATForDst(local, t); err != nil {
 					return nil, fmt.Errorf("error setting up SNAT rule: %w", err)
 				}
 			}