mm: vmscan: use a new flag to indicate shrinker is registered

Currently registered shrinker is indicated by non-NULL
shrinker->nr_deferred.  This approach is fine with nr_deferred at the
shrinker level, but the following patches will move MEMCG_AWARE
shrinkers' nr_deferred to memcg level, so their shrinker->nr_deferred
would always be NULL.  This would prevent the shrinkers from
unregistering correctly.

Remove SHRINKER_REGISTERING since we could check if shrinker is
registered successfully by the new flag.

Link: https://lkml.kernel.org/r/20210311190845.9708-9-shy828301@gmail.com


Signed-off-by: Yang Shi <shy828301@gmail.com>
Acked-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
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>