ucounts: move kfree() out of critical zone protected by ucounts_lock

Although kfree is a non-sleep function, it is possible to enter a long
chain of calls probabilistically, so it looks better to move kfree from
alloc_ucounts() out of the critical zone of ucounts_lock.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: MengEn Sun <[e-mail]>
Reviewed-by: YueHong Wu <[e-mail]>
Reviewed-by: Andrew Morton <[e-mail]>
Cc: Andrei Vagin <[e-mail]>
Cc: Joel Granados <[e-mail]>
Cc: Thomas Weißschuh <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

kernel/ucount.c

···
 struct ucounts *alloc_ucounts(struct user_namespace *ns, kuid_t uid)
 {
 	struct hlist_head *hashent = ucounts_hashentry(ns, uid);
-	struct ucounts *ucounts, *new;
 	bool wrapped;
+	struct ucounts *ucounts, *new = NULL;
 
 	spin_lock_irq(&ucounts_lock);
 	ucounts = find_ucounts(ns, uid, hashent);
···
 
 		spin_lock_irq(&ucounts_lock);
 		ucounts = find_ucounts(ns, uid, hashent);
-		if (ucounts) {
-			kfree(new);
-		} else {
+		if (!ucounts) {
 			hlist_add_head(&new->node, hashent);
 			get_user_ns(new->ns);
 			spin_unlock_irq(&ucounts_lock);
 			return new;
 		}
 	}
+
 	wrapped = !get_ucounts_or_wrap(ucounts);
 	spin_unlock_irq(&ucounts_lock);
+	kfree(new);
 	if (wrapped) {
 		put_ucounts(ucounts);
 		return NULL;