KVM: nSVM: Triple fault if restore host CR3 fails on nested #VMEXIT

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

KVM: nSVM: Triple fault if restore host CR3 fails on nested #VMEXIT

If loading L1's CR3 fails on a nested #VMEXIT, nested_svm_vmexit()
returns an error code that is ignored by most callers, and continues to
run L1 with corrupted state. A sane recovery is not possible in this
case, and HW behavior is to cause a shutdown. Inject a triple fault
instead, and do not return early from nested_svm_vmexit(). Continue
cleaning up the vCPU state (e.g. clear pending exceptions), to handle
the failure as gracefully as possible.

From the APM:

Upon #VMEXIT, the processor performs the following actions in order to
return to the host execution context:

...

if (illegal host state loaded, or exception while loading host state)
shutdown
else
execute first host instruction following the VMRUN

Remove the return value of nested_svm_vmexit(), which is mostly
unchecked anyway.