mm/vmscan.c: add a new member reclaim_state in struct shrink_control

Patch series "mm/vmscan: calculate reclaimed slab in all reclaim paths".

This patchset is to fix the issues in doing shrink slab.

There're six different reclaim paths by now,
 - kswapd reclaim path
 - node reclaim path
 - hibernate preallocate memory reclaim path
 - direct reclaim path
 - memcg reclaim path
 - memcg softlimit reclaim path

The slab caches reclaimed in these paths are only calculated in the
above three paths.  The issues are detailed explained in patch #2.  We
should calculate the reclaimed slab caches in every reclaim path.  In
order to do it, the struct reclaim_state is placed into the struct
shrink_control.

In node reclaim path, there'is another issue about shrinking slab, which
is adressed in "mm/vmscan: shrink slab in node reclaim"
(https://lore.kernel.org/linux-mm/1559874946-22960-1-git-send-email-laoar.shao@gmail.com/).

This patch (of 2):

The struct reclaim_state is used to record how many slab caches are
reclaimed in one reclaim path.  The struct shrink_control is used to
control one reclaim path.  So we'd better put reclaim_state into
shrink_control.

[laoar.shao@gmail.com: remove reclaim_state assignment from __perform_reclaim()]
Link: http://lkml.kernel.org/r/1561381582-13697-1-git-send-email-laoar.shao@gmail.com
Link: http://lkml.kernel.org/r/1561112086-6169-2-git-send-email-laoar.shao@gmail.com
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>