mm/swap: clarify unreachable cond_resched() in swap_reclaim_full_clusters

Commit 66366d291f ("mm/swap: add cond_resched() in
swap_reclaim_full_clusters to prevent softlockup") added cond_resched() to
prevent soft lockups during heavy swap reclaim.

Currently, there are two call sites for this function:
1. swap_reclaim_work(): called with force=true in a workqueue context.
2. cluster_alloc_swap_entry(): called with force=false, holding a
   local_lock and potentially a global cluster spinlock (atomic context).

In the second case, calling cond_resched() would normally cause a
"scheduling while atomic" bug.  However, it is currently safe because when
force=false, 'to_scan' is initialized to 1.  The loop decrements it to 0
and breaks before ever reaching cond_resched().

This implicit dependency is hard to notice and recently triggered a false
positive in AI Sashiko review.

Add a comment to explicitly clarify that cond_resched() is unreachable in
atomic contexts.  This improves readability and prevents future misuse if
the loop logic or 'to_scan' initialization is modified.

Link: https://sashiko.dev/#/patchset/20260713025644.170839-1-youngjun.park@lge.com?part=4
Link: https://lore.kernel.org/20260713045014.219653-1-youngjun.park@lge.com
Signed-off-by: Youngjun Park <youngjun.park@lge.com>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/swapfile.c

@@ -1072,6 +1072,12 @@ static void swap_reclaim_full_clusters(struct swap_info_struct *si, bool force)
 		swap_cluster_unlock(ci);
 		if (to_scan <= 0)
 			break;
+
+		/*
+		 * When 'force' is false, 'to_scan' is initialized to 1.
+		 * The loop breaks above, making this cond_resched() unreachable
+		 * in atomic contexts.
+		 */
 		cond_resched();
 	}
 }