blk-mq: constify struct blk_mq_ops

Constify all instances of blk_mq_ops, as they are never modified.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/drivers/block/null_blk.c b/drivers/block/null_blk.c
index 444f8d8..f93906f 100644
--- a/drivers/block/null_blk.c
+++ b/drivers/block/null_blk.c
@@ -398,7 +398,7 @@ static int null_init_hctx(struct blk_mq_hw_ctx *hctx, void *data,
 	return 0;
 }
 
-static struct blk_mq_ops null_mq_ops = {
+static const struct blk_mq_ops null_mq_ops = {
 	.queue_rq       = null_queue_rq,
 	.init_hctx	= null_init_hctx,
 	.complete	= null_softirq_done_fn,