mm: shrinkers: convert shrinker_rwsem to mutex

mm: shrinkers: convert shrinker_rwsem to mutex

Now there are no readers of shrinker_rwsem, so we can simply replace it
with mutex lock.

Link: https://lkml.kernel.org/r/20230313112819.38938-9-zhengqi.arch@bytedance.com
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Kirill Tkhai <tkhai@ya.ru>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Christian König <christian.koenig@amd.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Sultan Alsawaf <sultan@kerneltoast.com>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>