mq-deadline: covert to use request_queue->async_depth

mq-deadline: covert to use request_queue->async_depth

In downstream kernel, we test with mq-deadline with many fio workloads, and
we found a performance regression after commit 39823b47bbd4
("block/mq-deadline: Fix the tag reservation code") with following test:

[global]
rw=randread
direct=1
ramp_time=1
ioengine=libaio
iodepth=1024
numjobs=24
bs=1024k
group_reporting=1
runtime=60

[job1]
filename=/dev/sda

Root cause is that mq-deadline now support configuring async_depth,
although the default value is nr_request, however the minimal value is
1, hence min_shallow_depth is set to 1, causing wake_batch to be 1. For
consequence, sbitmap_queue will be waken up after each IO instead of
8 IO.

In this test case, sda is HDD and max_sectors is 128k, hence each
submitted 1M io will be splited into 8 sequential 128k requests, however
due to there are 24 jobs and total tags are exhausted, the 8 requests are
unlikely to be dispatched sequentially, and changing wake_batch to 1
will make this much worse, accounting blktrace D stage, the percentage
of sequential io is decreased from 8% to 0.8%.

Fix this problem by converting to request_queue->async_depth, where
min_shallow_depth is set each time async_depth is updated.

Noted elevator attribute async_depth is now removed, queue attribute
with the same name is used instead.

Fixes: 39823b47bbd4 ("block/mq-deadline: Fix the tag reservation code")
Signed-off-by: Yu Kuai <yukuai@fnnas.com>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>