quota: Cleanup list iteration in dqcache_shrink_scan()

Use list_first_entry() and list_empty() instead of opencoded variants.
Reviewed-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/quota/dquot.c

@@ -711,21 +711,18 @@ EXPORT_SYMBOL(dquot_quota_sync);
 static unsigned long
 dqcache_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 {
-	struct list_head *head;
 	struct dquot *dquot;
 	unsigned long freed = 0;
 
 	spin_lock(&dq_list_lock);
-	head = free_dquots.next;
-	while (head != &free_dquots && sc->nr_to_scan) {
-		dquot = list_entry(head, struct dquot, dq_free);
+	while (!list_empty(&free_dquots) && sc->nr_to_scan) {
+		dquot = list_first_entry(&free_dquots, struct dquot, dq_free);
 		remove_dquot_hash(dquot);
 		remove_free_dquot(dquot);
 		remove_inuse(dquot);
 		do_destroy_dquot(dquot);
 		sc->nr_to_scan--;
 		freed++;
-		head = free_dquots.next;
 	}
 	spin_unlock(&dq_list_lock);
 	return freed;