mm/swap: add cond_resched() in swap_reclaim_full_clusters to prevent softlockup

We hit a real softlockup in an internal stress test environment.  The
workload was LTP memory/swap stress on a large arm64 machine, with 320
CPUs, about 1TB memory and an 8.6GB swap device.  The system was under
heavy load and the swap device had a large number of full clusters.  The
softlockup was triggered during a stress test after about 3 days.

So, add periodic cond_resched() calls during large full_clusters
reclaim operations to prevent softlockup issues.

Detailed call trace as follow:

PID: 3817773  TASK: ffff0883bb28b780  CPU: 48   COMMAND: "kworker/48:7"
   #0 [ffff800080183d10] __crash_kexec at ffffa4c1361e5de4
   #1 [ffff800080183d90] panic at ffffa4c1360d5e9c
   #2 [ffff800080183e20] watchdog_timer_fn at ffffa4c136231fa8
   ...
  #16 [ffff8000c4ad3cb0] swap_cache_del_folio at ffffa4c1363e1614
  #17 [ffff8000c4ad3ce0] __try_to_reclaim_swap at ffffa4c1363e4bfc
  #18 [ffff8000c4ad3d40] swap_reclaim_full_clusters at ffffa4c1363e5474
  #19 [ffff8000c4ad3da0] swap_reclaim_work at ffffa4c1363e550c
  #20 [ffff8000c4ad3dc0] process_one_work at ffffa4c136102edc
  #21 [ffff8000c4ad3e10] worker_thread at ffffa4c136103398
  #22 [ffff8000c4ad3e70] kthread at ffffa4c13610d95c

Link: https://lore.kernel.org/20260506130919.2298807-1-kerayhuang@tencent.com
Fixes: 5168a68eb7 ("mm, swap: avoid over reclaim of full clusters")
Signed-off-by: Zijiang Huang <kerayhuang@tencent.com>
Reviewed-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Hao Peng <flyingpeng@tencent.com>
Reviewed-by: albinwyang <albinwyang@tencent.com>
Reviewed-by: Baoquan He <baoquan.he@linux.dev>
Acked-by: Chris Li <chrisl@kernel.org>
Cc: Barry Song <baohua@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Youngjun Park <youngjun.park@lge.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/swapfile.c

@@ -1054,6 +1054,7 @@ static void swap_reclaim_full_clusters(struct swap_info_struct *si, bool force)
 		swap_cluster_unlock(ci);
 		if (to_scan <= 0)
 			break;
+		cond_resched();
 	}
 }