ublk ADD_DEV repro

Minimal reproducer for a ublk_drv crash seen on the GitHub Actions ubuntu-24.04 amd64 runner kernel:

6.17.0-1015-azure #15~24.04.1-Ubuntu
RIP: ublk_init_queues+0x4e/0x1e0 [ublk_drv]

The program issues one UBLK_U_CMD_ADD_DEV using raw io_uring (IORING_OP_URING_CMD, SQE128), then deletes the device if creation succeeds. It does not depend on ublk-go.

Run

sudo modprobe ublk_drv
make
sudo ./ublk_add_dev_repro

Known results:

• 6.17.0-1015-azure (ubuntu-24.04 GitHub hosted runner): kernel Oops in ublk_init_queues; the command never completes.
• 6.8.0-1052-azure (ubuntu-22.04 GitHub hosted runner): passes.
• 6.17.0-7-generic: passes.

See REPORT.md for the short bug report text.