t/io_uring: pre-calculate per-file depth

t/io_uring: pre-calculate per-file depth

Rather than do this math every time, just calculate it at setup time
and use that.

Note that there's something wonky with how the depths are distributed,
it's quite common to see the first file eat the majority of the queue
depth, causing multiple file runs to be both slower than they should be,
and with very unevenly distributed depths. To be investigated.

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