null_blk: unalign call_single_data

Commit 966a9671 randomly added alignment to this structure, but
it's actually detrimental to performance of null_blk. Test case:

Running on both the home and remote node shows a ~5% degradation
in performance.

While in there, move blk_status_t to the hole after the integer tag
in the nullb_cmd structure. After this patch, we shrink the size
from 192 to 152 bytes.

Fixes: 966a9671 ("smp: Avoid using two cache lines for struct call_single_data")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk.c

@@ -35,13 +35,13 @@ static inline u64 mb_per_tick(int mbps)
 struct nullb_cmd {
 	struct list_head list;
 	struct llist_node ll_list;
-	call_single_data_t csd;
+	struct __call_single_data csd;
 	struct request *rq;
 	struct bio *bio;
 	unsigned int tag;
+	blk_status_t error;
 	struct nullb_queue *nq;
 	struct hrtimer timer;
-	blk_status_t error;
 };
 
 struct nullb_queue {