t/io_uring: fix size confusion in allocate_mem()

t/io_uring: fix size confusion in allocate_mem()

It should always be using the size passed in as to what amount of memory
should be allocated, not the individual IO size. This didn't matter
previously, as each buffer was allocated independently, but it matters
now where a single region will cover all IO buffers.

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