mm/swap: clarify unreachable cond_resched() in swap_reclaim_full_clusters

Commit 66366d291f66 ("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.

---
 mm/swapfile.c | 6 ++++++
 1 file changed, 6 insertions(+)

index 72952491e9cf..00b5fd8683e5 100644
@@ -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();
 	}
 }

2.34.1