bcache: do not check NULL pointer before calling kmem_cache_destroy

kmem_cache_destroy() is safe for NULL pointer as input, the NULL pointer
checking is unncessary. This patch just removes the NULL pointer checking
to make code simpler.
Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Shenghui Wang <shhuiw@foxmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/request.c

@@ -1367,8 +1367,7 @@ void bch_flash_dev_request_init(struct bcache_device *d)
 
 void bch_request_exit(void)
 {
-	if (bch_search_cache)
-		kmem_cache_destroy(bch_search_cache);
+	kmem_cache_destroy(bch_search_cache);
 }
 
 int __init bch_request_init(void)