mm/swap: fix wrong plist empty check in swap_alloc_slow()

swap_alloc_slow() was checking `si->avail_list` instead of
`next->avail_list` when verifying if the next swap device is still in the
list, which could cause unnecessary restarts during allocation.

Link: https://lkml.kernel.org/r/20251119114136.594108-1-youngjun.park@lge.com
Fixes: 8e689f8 ("mm/swap: do not choose swap device according to numa node")
Signed-off-by: Youngjun Park <youngjun.park@lge.com>
Acked-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: youngjuni

mm/swapfile.c

@@ -1374,7 +1374,7 @@ static bool swap_alloc_slow(swp_entry_t *entry,
 		 * still in the swap_avail_head list then try it, otherwise
 		 * start over if we have not gotten any slots.
 		 */
-		if (plist_node_empty(&si->avail_list))
+		if (plist_node_empty(&next->avail_list))
 			goto start_over;
 	}
 	spin_unlock(&swap_avail_lock);