ublk: factor out ublk_start_io() helper

In preparation for calling it from outside ublk_dispatch_req(), factor
out the code responsible for setting up an incoming ublk I/O request.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250430225234.2676781-7-csander@purestorage.com


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