io_uring: clamp CQ size to SQ size

io_uring: clamp CQ size to SQ size

By default, io_uring uses twice as big a CQ ring as the SQ ring. That's
to help with cases where completions can come in unexpectedly. This is not
the case for storage IO, so just clamp the CQ size to save a bit of memory
on the CQEs and CQ ring.

Signed-off-by: Jens Axboe <axboe@kernel.dk>