mm/swap: remove unneeded return value of free_swap_slot

The return value of free_swap_slot is always 0 and also ignored now.
Remove it to clean up the code.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Miaohe Lin <[e-mail]>
Reviewed-by: David Hildenbrand <[e-mail]>
Reviewed-by: Oscar Salvador <[e-mail]>
Cc: Alistair Popple <[e-mail]>
Cc: David Howells <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: Naoya Horiguchi <[e-mail]>
Cc: NeilBrown <[e-mail]>
Cc: Peter Xu <[e-mail]>
Cc: Suren Baghdasaryan <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/swap_slots.h

···
 void disable_swap_slots_cache_lock(void);
 void reenable_swap_slots_cache_unlock(void);
 void enable_swap_slots_cache(void);
-int free_swap_slot(swp_entry_t entry);
+void free_swap_slot(swp_entry_t entry);
 
 extern bool swap_slot_cache_enabled;
 

mm/swap_slots.c

···
 	return cache->nr;
 }
 
-int free_swap_slot(swp_entry_t entry)
+void free_swap_slot(swp_entry_t entry)
 {
 	struct swap_slots_cache *cache;
 
···
 direct_free:
 		swapcache_free_entries(&entry, 1);
 	}
-
-	return 0;
 }
 
 swp_entry_t folio_alloc_swap(struct folio *folio)