selftests: ublk: add ublk_io_buf_idx() for returning io buffer index

selftests: ublk: add ublk_io_buf_idx() for returning io buffer index

Since UBLK_F_PER_IO_DAEMON is added, io buffer index may depend on current
thread because the common way is to use per-pthread io_ring_ctx for issuing
ublk uring_cmd.

Add one helper for returning io buffer index, so we can hide the buffer
index implementation details for target code.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>