io_uring: refactor io_iopoll_req_issued

A simple refactoring of io_iopoll_req_issued(), move in_async inside so
we don't pass it around and save on double checking it.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/1513bfde4f0c835be25ac69a82737ab0668d7665.1623634181.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>