driver: preserve device identity across PCI hotplug

[amd-vserbu]

Summary

This enables long-running Docker containers to keep using a V80 across SLASH remove/rescan and reset cycles when the device is passed through with docker run --device.

Docker records a device's major/minor number when the container is created. The previous miscdevice implementation allocated a new dynamic minor whenever PF1 or PF2 was reprobed. After a PCI remove/rescan, the same /dev path could therefore return on the host with a different device number, while the container retained the old device node and device-cgroup permission. Restarting the container was the only way to pass the replacement device through.

The driver now reserves one character-device range for the lifetime of slash.ko and assigns deterministic minors:

• minor 0: slash_hotplug
• minor 2*N+1: slash_ctl<N>
• minor 2*N+2: slash_qdma_ctl<N>

PF1 and PF2 are paired by their board BDF and retain the same N until the module is unloaded. Reprobed devices consequently return with the same /dev path and dev_t, so a device node already present in a container opens the new device instance without recreating the container.

For example, a container can be started once with:

docker run \
  --device=/dev/slash_hotplug \
  --device=/dev/slash_ctl0 \
  --device=/dev/slash_qdma_ctl0 \
  ...

and continue using those paths after a host-side remove/rescan.

Other changes

• Move SLASH devices from /sys/class/misc to a dedicated /sys/class/slash class.
• Add safe character-device lifetime handling: an fd opened before removal remains tied to the removed instance and returns ENODEV; it never silently rebinds to the rescanned device.
• Report the PF1 BDF through SLASH_QDMA_IOCTL_INFO, allowing userspace to verify CTL/QDMA pairing without depending on sysfs naming.
• Simplify vrtd and v80-smi discovery around the stable slash_ctl<N>/slash_qdma_ctl<N> pairing.
• Support old and new kernel signatures for class_create() and the class devnode callback.
• Update the kernel ABI documentation and driver, libslash, vrtd, and v80-smi tests for the new class and device-number layout.