mm: shrinkers: add scan interface for shrinker debugfs

Add a scan interface which allows to trigger scanning of a particular
shrinker and specify memcg and numa node.  It's useful for testing,
debugging and profiling of a specific scan_objects() callback.  Unlike
alternatives (creating a real memory pressure and dropping caches via
/proc/sys/vm/drop_caches) this interface allows to interact with only one
shrinker at once.  Also, if a shrinker is misreporting the number of
objects (as some do), it doesn't affect scanning.

[roman.gushchin@linux.dev: improve typing, fix arg count checking]
  Link: https://lkml.kernel.org/r/YpgKttTowT22mKPQ@carbon
[akpm@linux-foundation.org: fix arg count checking]
Link: https://lkml.kernel.org/r/20220601032227.4076670-7-roman.gushchin@linux.dev


Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
Acked-by: Muchun Song <songmuchun@bytedance.com>
Cc: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Hillf Danton <hdanton@sina.com>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>