block: ublk: switch to ioctl command encoding

All ublk commands(control, IO) should have taken ioctl command encoding
from the beginning, because ioctl command encoding defines each code
uniquely, so driver can figure out wrong command sent from userspace
easily; 2) it might help security subsystem for audit uring cmd[1].

Unfortunately we didn't do that way, and it could be one lesson for
ublk driver.

So switch to ioctl command encoding now, we still support commands encoded
in old way, but they become legacy definition. Any new command should take
ioctl encoding.

See ublksrv code for switching to ioctl command encoding in [2].

[1] https://lore.kernel.org/io-uring/CAHC9VhSVzujW9LOj5Km80AjU0EfAuukoLrxO6BEfnXeK_s6bAg@mail.gmail.com/
[2] https://github.com/ming1/ubdsrv/commits/ioctl_cmd_encoding

Cc: Christoph Hellwig <hch@infradead.org>
Cc: Ken Kurematsu <k.kurematsu@nskint.co.jp>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230418131810.855959-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>