mm: pagevec: defer deciding which LRU to add a page to until pagevec drain time

mark_page_accessed() cannot activate an inactive page that is located on
an inactive LRU pagevec.  Hints from filesystems may be ignored as a
result.  In preparation for fixing that problem, this patch removes the
per-LRU pagevecs and leaves just one pagevec.  The final LRU the page is
added to is deferred until the pagevec is drained.

This means that fewer pagevecs are available and potentially there is
greater contention on the LRU lock.  However, this only applies in the
case where there is an almost perfect mix of file, anon, active and
inactive pages being added to the LRU.  In practice I expect that we are
adding stream of pages of a particular time and that the changes in
contention will barely be measurable.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Jan Kara <jack@suse.cz>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Alexey Lyahkov <alexey.lyashkov@gmail.com>
Cc: Andrew Perepechko <anserper@ya.ru>
Cc: Robin Dong <sanbai@taobao.com>
Cc: Theodore Tso <tytso@mit.edu>
Cc: Hugh Dickins <hughd@google.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Bernd Schubert <bernd.schubert@fastmail.fm>
Cc: David Howells <dhowells@redhat.com>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>