KVM: SVM: Clear current_vmcb during vCPU free for all *possible* CPUs

When freeing a vCPU and thus its VMCB, clear current_vmcb for all possible
CPUs, not just online CPUs, as it's theoretically possible a CPU could go
offline and come back online in conjunction with KVM reusing the page for
a new VMCB.

Link: https://lore.kernel.org/all/[email protected]
Fixes: fd65d31 ("kvm: svm: Ensure an IBPB on all affected CPUs when freeing a vmcb")
Cc: [email protected]
Cc: Jim Mattson <[email protected]>
Signed-off-by: Yosry Ahmed <[email protected]>
[sean: split to separate patch, write changelog]
Signed-off-by: Sean Christopherson <[email protected]>

Author: yosrym93

arch/x86/kvm/svm/svm.c

@@ -1496,7 +1496,7 @@ static void svm_clear_current_vmcb(struct vmcb *vmcb)
 {
 	int i;
 
-	for_each_online_cpu(i)
+	for_each_possible_cpu(i)
 		cmpxchg(per_cpu_ptr(&svm_data.current_vmcb, i), vmcb, NULL);
 }