t/io_uring: allow multiple IO threads

t/io_uring: allow multiple IO threads

If you do:

t/io_uring -n2 /dev/dev1 /dev/dev2

then t/io_uring will create two IO threads, and each one will get
a file/device assigned. In the above example, thread 1 will run
on dev1, thread 2 on dev2.

Note that for now, you'll need at least as many files as threads.
Adding support for adding the same file set over the specified
threads (if we have less files than threads) is left as an
exercise for the reader. You know where to send the patches.

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