md: add support for REQ_NOWAIT

commit 021a2446 ("block: add QUEUE_FLAG_NOWAIT") added support
for checking whether a given bdev supports handling of REQ_NOWAIT or not.
Since then commit 6abc4946

 ("dm: add support for REQ_NOWAIT and enable
it for linear target") added support for REQ_NOWAIT for dm. This uses
a similar approach to incorporate REQ_NOWAIT for md based bios.

This patch was tested using t/io_uring tool within FIO. A nvme drive
was partitioned into 2 partitions and a simple raid 0 configuration
/dev/md0 was created.

md0 : active raid0 nvme4n1p1[1] nvme4n1p2[0]
      937423872 blocks super 1.2 512k chunks

Before patch:

$ ./t/io_uring /dev/md0 -p 0 -a 0 -d 1 -r 100

Running top while the above runs:

$ ps -eL | grep $(pidof io_uring)

  38396   38396 pts/2    00:00:00 io_uring
  38396   38397 pts/2    00:00:15 io_uring
  38396   38398 pts/2    00:00:13 iou-wrk-38397

We can see iou-wrk-38397 io worker thread created which gets created
when io_uring sees that the underlying device (/dev/md0 in this case)
doesn't support nowait.

After patch:

$ ./t/io_uring /dev/md0 -p 0 -a 0 -d 1 -r 100

Running top while the above runs:

$ ps -eL | grep $(pidof io_uring)

  38341   38341 pts/2    00:10:22 io_uring
  38341   38342 pts/2    00:10:37 io_uring

After running this patch, we don't see any io worker thread
being created which indicated that io_uring saw that the
underlying device does support nowait. This is the exact behaviour
noticed on a dm device which also supports nowait.

For all the other raid personalities except raid0, we would need
to train pieces which involves make_request fn in order for them
to correctly handle REQ_NOWAIT.

Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Vishal Verma <vverma@digitalocean.com>
Signed-off-by: Song Liu <song@kernel.org>