vmscan: handle isolated pages with lru lock released

When shrinking inactive lru list, isolated pages are queued on locally
private list, so the lock-hold time could be reduced if pages are counted
without lock protection.

To achieve that, firstly updating reclaim stat is delayed until the
putback stage, after reacquiring the lru lock.

Secondly, operations related to vm and zone stats are now proteced with
preemption disabled as they are per-cpu operations.

Signed-off-by: Hillf Danton <dhillf@gmail.com>
Acked-by: Hugh Dickins <hughd@google.com>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>