mm: remove UP version of lru_add_drain_all()

Currently, lru_add_drain_all() has two version.
(1) use schedule_on_each_cpu()
(2) don't use schedule_on_each_cpu()

Gerald Schaefer reported it doesn't work well on SMP (not NUMA) S390
machine.

offline_pages() calls lru_add_drain_all() followed by drain_all_pages().
While drain_all_pages() works on each cpu, lru_add_drain_all() only runs
on the current cpu for architectures w/o CONFIG_NUMA. This let us run
into the BUG_ON(!PageBuddy(page)) in __offline_isolated_pages() during
memory hotplug stress test on s390. The page in question was still on the
pcp list, because of a race with lru_add_drain_all() and drain_all_pages()
on different cpus.

Actually, Almost machine has CONFIG_UNEVICTABLE_LRU=y. Then almost machine use
(1) version lru_add_drain_all although the machine is UP.

Then this ifdef is not valueable.
simple removing is better.

Signed-off-by: KOSAKI Motohiro <[e-mail]>
Cc: Christoph Lameter <[e-mail]>
Cc: Lee Schermerhorn <[e-mail]>
Acked-by: Gerald Schaefer <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/swap.c

···
 	put_cpu();
 }
 
-#if defined(CONFIG_NUMA) || defined(CONFIG_UNEVICTABLE_LRU)
 static void lru_add_drain_per_cpu(struct work_struct *dummy)
 {
 	lru_add_drain();
···
 {
 	return schedule_on_each_cpu(lru_add_drain_per_cpu);
 }
-
-#else
-
-/*
- * Returns 0 for success
- */
-int lru_add_drain_all(void)
-{
-	lru_add_drain();
-	return 0;
-}
-#endif
 
 /*
  * Batched page_cache_release().  Decrement the reference count on all the