Fix kubeadm upgrade plan issue with FQDN nodes names

the fix introduced in kubernetes#110634 also introduces a bug preventing `kubeadm
upgrade plan` from running on nodes having a different `os.Hostname()`
than node name. concretely, for a node `titi.company.ch`,
`os.Hostname()` will return `titi`, while the full node name is actually
`titi.company.ch`. this simple fix uses the `cfg.NodeRegistration.Name`
instead, which fixes the issue on my nodes with a FQDN node name

Keep previous hostname retrieval as fallback for dupURL CRI fix

cmd/kubeadm/app/cmd/upgrade/common.go

@@ -232,9 +232,14 @@ func enforceRequirements(flags *applyPlanFlags, args []string, dryRun bool, upgr
 	if dupURLScheme {
 		socket := strings.ReplaceAll(cfg.NodeRegistration.CRISocket, kubeadmapiv1.DefaultContainerRuntimeURLScheme+"://", "")
 		cfg.NodeRegistration.CRISocket = kubeadmapiv1.DefaultContainerRuntimeURLScheme + "://" + socket
-		hostname, err := os.Hostname()
-		if err != nil {
-			return nil, nil, nil, errors.Wrapf(err, "failed to get hostname")
+		var hostname string
+		if len(cfg.NodeRegistration.Name) > 0 {
+			hostname = cfg.NodeRegistration.Name
+		} else {
+			hostname, err = os.Hostname()
+			if err != nil {
+				return nil, nil, nil, errors.Wrapf(err, "failed to get hostname")
+			}
 		}
 		klog.V(2).Infof("ensuring that Node %q has a CRI socket annotation with correct URL scheme %q", hostname, cfg.NodeRegistration.CRISocket)
 		if err := patchnodephase.AnnotateCRISocket(client, hostname, cfg.NodeRegistration.CRISocket); err != nil {