uring_cmd may never complete, such as ublk, in which uring cmd isn't
completed until one new block request is coming from ublk block device.

Add cancelable uring_cmd to provide mechanism to driver for cancelling
pending commands in its own way.

Add API of io_uring_cmd_mark_cancelable() for driver to mark one command as
cancelable, then io_uring will cancel this command in
io_uring_cancel_generic(). ->uring_cmd() callback is reused for canceling
command in driver's way, then driver gets notified with the cancelling
from io_uring.

Add API of io_uring_cmd_get_task() to help driver cancel handler
deal with the canceling.
Reviewed-by: Gabriel Krisman Bertazi <krisman@suse.de>
Suggested-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>