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/20220509131416.17553-5-linmiaohe@huawei.comSigned-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: NeilBrown <neilb@suse.de>
Cc: Peter Xu <peterx@redhat.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/swap_slots.h

@@ -24,7 +24,7 @@ struct swap_slots_cache {
 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

@@ -269,7 +269,7 @@ static int refill_swap_slots_cache(struct swap_slots_cache *cache)
 	return cache->nr;
 }
 
-int free_swap_slot(swp_entry_t entry)
+void free_swap_slot(swp_entry_t entry)
 {
 	struct swap_slots_cache *cache;
 
@@ -297,8 +297,6 @@ int free_swap_slot(swp_entry_t entry)
 direct_free:
 		swapcache_free_entries(&entry, 1);
 	}
-
-	return 0;
 }
 
 swp_entry_t folio_alloc_swap(struct folio *folio)