io_uring: add infra for uring_cmd completion in submitter-task.

io_uring: add infra for uring_cmd completion in submitter-task.

Completion of a uring_cmd ioctl may involve referencing certain
ioctl-specific fields, requiring original submitter context.
Export an API that driver can use for this purpose.
The API facilitates reusing task-work infra of io_uring, while driver
gets to implement cmd-specific handling in a callback.

Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
Signed-off-by: Anuj Gupta <anuj20.g@samsung.com>
Link: https://lore.kernel.org/r/20210805125539.66958-2-joshi.k@samsung.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>