io_uring: fix stale comment and a few typos

- Fix a few typos found while reading the code.

- Fix stale io_get_sqring comment referencing s->sqe, the 's' parameter
  was renamed to 'req', but the comment still holds.

Signed-off-by: Brian Gianforcaro <b.gianfo@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>