Drivers: hv: vmbus: Fix potential crash on module unload

In the Linux kernel, the following vulnerability has been resolved:

Drivers: hv: vmbus: Fix potential crash on module unload

The vmbus driver relies on the panic notifier infrastructure to perform

some operations when a panic event is detected. Since vmbus can be built

as module, it is required that the driver handles both registering and

unregistering such panic notifier callback.

After commit 74347a99e73a ("x86/Hyper-V: Unload vmbus channel in hv panic callback")

though, the panic notifier registration is done unconditionally in the module

initialization routine whereas the unregistering procedure is conditionally

guarded and executes only if HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE capability

is set.

This patch fixes that by unconditionally unregistering the panic notifier

in the module's exit routine as well.