KVM: VMX: Prevent RSB underflow before vmenter

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

KVM: VMX: Prevent RSB underflow before vmenter

On VMX, there are some balanced returns between the time the guest's

SPEC_CTRL value is written, and the vmenter.

Balanced returns (matched by a preceding call) are usually ok, but it's

at least theoretically possible an NMI with a deep call stack could

empty the RSB before one of the returns.

For maximum paranoia, don't allow *any* returns (balanced or otherwise)

between the SPEC_CTRL write and the vmenter.

[ bp: Fix 32-bit build. ]