engines/io_uring: consolidate fio_ioring_cqring_reap() arguments

engines/io_uring: consolidate fio_ioring_cqring_reap() arguments

fio_ioring_cqring_reap() takes both an events and a max argument and
will return up to events - max CQEs. Only one of the two callers passes
an existing events count. So remove the events argument and have
fio_ioring_getevents() pass events - max instead. This simplifies the
function signature and avoids an addition inside the loop over CQEs.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>