io_uring: correct size of overflow CQE calculation

io_uring: correct size of overflow CQE calculation

If a 32b CQE is required, don't double the size of the overflow struct,
just add the size of the io_uring_cqe addition that is needed. This
avoids allocating too much memory, as the io_overflow_cqe size includes
the list member required to queue them too.

Fixes: e26dca67fde1 ("io_uring: add support for IORING_SETUP_CQE_MIXED")
Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>