mm/shrinker: simplify shrinker_memcg_alloc() using guard()

mm/shrinker: simplify shrinker_memcg_alloc() using guard()

Use guard(mutex) to automatically handle shrinker_mutex locking and
unlocking in shrinker_memcg_alloc().  This removes the explicit
mutex_unlock() call, the goto-based error path, and the redundant ret
variable, resulting in cleaner and more concise code.

Link: https://lore.kernel.org/20260513075214.2655710-1-18810879172@163.com
Signed-off-by: wangxuewen <wangxuewen@kylinos.cn>
Acked-by: Muchun Song <muchun.song@linux.dev>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Xuewen Wang <wangxuewen@kylinos.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>