mnt: Use hlist_move_list in namespace_unlock

Small cleanup to make the code more readable and maintainable.
Signed-off-by: NEric Biederman <ebiederm@xmission.com>

fs/namespace.c

@@ -1298,17 +1298,15 @@ static HLIST_HEAD(unmounted);	/* protected by namespace_sem */
 
 static void namespace_unlock(void)
 {
-	struct hlist_head head = unmounted;
+	struct hlist_head head;
 
-	if (likely(hlist_empty(&head))) {
-		up_write(&namespace_sem);
-		return;
-	}
+	hlist_move_list(&unmounted, &head);
 
-	head.first->pprev = &head.first;
-	INIT_HLIST_HEAD(&unmounted);
 	up_write(&namespace_sem);
 
+	if (likely(hlist_empty(&head)))
+		return;
+
 	synchronize_rcu();
 
 	group_pin_kill(&head);