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 <kosaki.motohiro@jp.fujitsu.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
Acked-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>