Use the min() macro to simplify the shrinker_debugfs_scan_write() function
and improve its readability.

Link: https://lkml.kernel.org/r/20240820042254.99115-2-thorsten.blum@toblux.comSigned-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>