io_uring: rename __io_submit_sqe()

__io_submit_sqe() is issuing requests, so call it as
such. Moreover, it ends by calling io_iopoll_req_issued().

Rename it and make terminology clearer.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>