engines/io_uring: remove loop over CQEs in fio_ioring_cqring_reap()

engines/io_uring: remove loop over CQEs in fio_ioring_cqring_reap()

Currently fio_ioring_cqring_reap() loops over each available CQE,
re-loading the tail index, incrementing local variables, and checking
whether the max requested CQEs have been seen.
Avoid the loop by computing the number of available CQEs as tail - head
and capping it to the requested max.

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