nfsd: helper for dup of possibly NULL string

Technically the initialization in the NULL case isn't even needed as the
only caller already has target zeroed out, but it seems safer to keep
copy_cred generic.
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4state.c

@@ -1857,15 +1857,24 @@ static void copy_clid(struct nfs4_client *target, struct nfs4_client *source)
 	target->cl_clientid.cl_id = source->cl_clientid.cl_id; 
 }
 
-static int copy_cred(struct svc_cred *target, struct svc_cred *source)
+int strdup_if_nonnull(char **target, char *source)
 {
-	if (source->cr_principal) {
-		target->cr_principal =
-				kstrdup(source->cr_principal, GFP_KERNEL);
-		if (target->cr_principal == NULL)
+	if (source) {
+		*target = kstrdup(source, GFP_KERNEL);
+		if (!*target)
 			return -ENOMEM;
 	} else
-		target->cr_principal = NULL;
+		*target = NULL;
+	return 0;
+}
+
+static int copy_cred(struct svc_cred *target, struct svc_cred *source)
+{
+	int ret;
+
+	ret = strdup_if_nonnull(&target->cr_principal, source->cr_principal);
+	if (ret)
+		return ret;
 	target->cr_flavor = source->cr_flavor;
 	target->cr_uid = source->cr_uid;
 	target->cr_gid = source->cr_gid;