io-wq: add support for uncancellable work

Not all work can be cancelled, some of it we may need to guarantee
that it runs to completion. Allow the caller to set IO_WQ_WORK_NO_CANCEL
on work that must not be cancelled. Note that the caller work function
must also check for IO_WQ_WORK_NO_CANCEL on work that is marked
IO_WQ_WORK_CANCEL.

Signed-off-by: Jens Axboe <axboe@kernel.dk>