mm, swap: avoid BUG_ON in relocate_cluster()

If allocation is racy with swapoff, we may call free_cluster for cluster
already in free list and trigger BUG_ON() as following:

Allocation                        Swapoff
cluster_alloc_swap_entry
 ...
 /* may get a free cluster with offset */
 offset = xxx;
 if (offset)
  ci = lock_cluster(si, offset);

                                  ...
                                   del_from_avail_list(p, true);
                                    si->flags &= ~SWP_WRITEOK;

  alloc_swap_scan_cluster(si, ci, ...)
   ...
   /* failed to alloc entry from free entry */
   if (!cluster_alloc_range(...))
    break;
   ...
   /* add back a free cluster */
   relocate_cluster(si, ci);
    if (!ci->count)
     free_cluster(si, ci);
      VM_BUG_ON(ci->flags == CLUSTER_FLAG_FREE);

To prevent the BUG_ON(), call free_cluster() for free cluster to move the
cluster to tail of list.

Check cluster is not free before calling free_cluster() in
relocate_cluster() to avoid BUG_ON().

Link: https://lkml.kernel.org/r/20250222160850.505274-4-shikemeng@huaweicloud.com
Fixes: 3b64477 ("mm, swap: reduce contention on device lock")
Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Kemeng Shi

mm/swapfile.c

@@ -653,7 +653,8 @@ static void relocate_cluster(struct swap_info_struct *si,
 		return;
 
 	if (!ci->count) {
-		free_cluster(si, ci);
+		if (ci->flags != CLUSTER_FLAG_FREE)
+			free_cluster(si, ci);
 	} else if (ci->count != SWAPFILE_CLUSTER) {
 		if (ci->flags != CLUSTER_FLAG_FRAG)
 			move_cluster(si, ci, &si->frag_clusters[ci->order],