Fix format string bug in GetPodMTU error logging (#3471)

Changed error log in GetPodMTU to use podMTU (string input) instead of
mtu (int result) when logging parse failures.

This fixes the format string mismatch that causes:

{"level":"error","ts":"2025-10-02T23:03:58.442+0200","caller":"routed-eni-cni-plugin/cni.go:163","msg":"Failed to parse pod MTU %!s(int=0): strconv.Atoi: parsing \"\": invalid syntax"}

in the output of "make unit-test".

Author: dcoppa

pkg/networkutils/network.go

@@ -1388,7 +1388,7 @@ func GetEthernetMTU() int {
 func GetPodMTU(podMTU string) int {
 	mtu, err := strconv.Atoi(podMTU)
 	if err != nil {
-		log.Errorf("Failed to parse pod MTU %s: %v", mtu, err)
+		log.Errorf("Failed to parse pod MTU %s: %v", podMTU, err)
 		return defaultMTU
 	}