mm/swap_slots.c: remove always zero and unused return value of enable_swap_slots_cache()

enable_swap_slots_cache() always return zero and its return value is just
ignored by the caller.  So make enable_swap_slots_cache() void.
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: https://lkml.kernel.org/r/20200924113554.50614-1-linmiaohe@huawei.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/swap_slots.h

@@ -23,7 +23,7 @@ struct swap_slots_cache {
 
 void disable_swap_slots_cache_lock(void);
 void reenable_swap_slots_cache_unlock(void);
-int enable_swap_slots_cache(void);
+void enable_swap_slots_cache(void);
 int free_swap_slot(swp_entry_t entry);
 
 extern bool swap_slot_cache_enabled;

mm/swap_slots.c

@@ -237,7 +237,7 @@ static int free_slot_cache(unsigned int cpu)
 	return 0;
 }
 
-int enable_swap_slots_cache(void)
+void enable_swap_slots_cache(void)
 {
 	mutex_lock(&swap_slots_cache_enable_mutex);
 	if (!swap_slot_cache_initialized) {
@@ -255,7 +255,6 @@ int enable_swap_slots_cache(void)
 	__reenable_swap_slots_cache();
 out_unlock:
 	mutex_unlock(&swap_slots_cache_enable_mutex);
-	return 0;
 }
 
 /* called with swap slot cache's alloc lock held */