mm/swap_slots.c: remove redundant NULL check

Fix below warnings reported by coccicheck:

  mm/swap_slots.c:197:3-9: WARNING: NULL check before some freeing functions is not needed.

Link: https://lkml.kernel.org/r/1611214229-11225-1-git-send-email-abaci-bugfix@linux.alibaba.com
Signed-off-by: Yang Li <abaci-bugfix@linux.alibaba.com>
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/swap_slots.c

@@ -193,8 +193,7 @@ static void drain_slots_cache_cpu(unsigned int cpu, unsigned int type,
 			cache->slots_ret = NULL;
 		}
 		spin_unlock_irq(&cache->free_lock);
-		if (slots)
-			kvfree(slots);
+		kvfree(slots);
 	}
 }