mm/vmscan.c: calculate reclaimed slab caches in all reclaim paths

There are 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.

There're some drawbacks if we don't calculate the reclaimed slab caches.

 - The sc->nr_reclaimed isn't correct if there're some slab caches
   relcaimed in this path.

 - The slab caches may be reclaimed thoroughly if there're lots of
   reclaimable slab caches and few page caches.

   Let's take an easy example for this case. If one memcg is full of
   slab caches and the limit of it is 512M, in other words there're
   approximately 512M slab caches in this memcg. Then the limit of the
   memcg is reached and the memcg reclaim begins, and then in this memcg
   reclaim path it will continuesly reclaim the slab caches until the
   sc->priority drops to 0. After this reclaim stops, you will find
   there're few slab caches left, which is less than 20M in my test
   case. While after this patch applied the number is greater than 300M
   and the sc->priority only drops to 3.

Link: http://lkml.kernel.org/r/1561112086-6169-3-git-send-email-laoar.shao@gmail.com
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: 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>