t/io_uring: register single buffer for whole IO region

t/io_uring: register single buffer for whole IO region

Rather than register N buffers for depth of N, just allocate a single
contig region and register that as a single buffer. That's more
efficient on the kernel side, as only a single resource need to be
managed.

Should have no functional changes.

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