KVM: VMX: Expose SSBD properly to guests, 4.9 supplement

Fix an additional misuse of X86_FEATURE_SSBD in
guest_cpuid_has_spec_ctrl().  This function was introduced in the
backport of SSBD support to 4.9 and is not present upstream, so it was
not fixed by commit 43462d90 "KVM: VMX: Expose SSBD properly to
guests."

Fixes: 52817587e706 ("x86/cpufeatures: Disentangle SSBD enumeration")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: kvm@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>