mm: vmscan: consolidate shrinker_maps handling code

mm: vmscan: consolidate shrinker_maps handling code

The shrinker map management is not purely memcg specific, it is at the
intersection between memory cgroup and shrinkers.  It's allocation and
assignment of a structure, and the only memcg bit is the map is being
stored in a memcg structure.  So move the shrinker_maps handling code
into vmscan.c for tighter integration with shrinker code, and remove the
"memcg_" prefix.  There is no functional change.

Link: https://lkml.kernel.org/r/20210311190845.9708-3-shy828301@gmail.com
Signed-off-by: Yang Shi <shy828301@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Acked-by: Roman Gushchin <guro@fb.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>