KVM: x86: fix wbinvd_dirty_mask use-after-free

KVM: x86: fix wbinvd_dirty_mask use-after-free

commit bd768e146624cbec7122ed15dead8daa137d909d upstream.

vcpu->arch.wbinvd_dirty_mask may still be used after freeing it,
corrupting memory. For example, the following call trace may set a bit
in an already freed cpu mask:
    kvm_arch_vcpu_load
    vcpu_load
    vmx_free_vcpu_nested
    vmx_free_vcpu
    kvm_arch_vcpu_free

Fix this by deferring freeing of wbinvd_dirty_mask.

Signed-off-by: Ido Yariv <ido@wizery.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>