x86/spec_ctrl: Fix determination of when to use IBRS

x86/spec_ctrl: Fix determination of when to use IBRS

The original version of this logic was:

    /*
     * On Intel hardware, we'd like to use retpoline in preference to
     * IBRS, but only if it is safe on this hardware.
     */
    else if ( boot_cpu_has(X86_FEATURE_IBRSB) )
    {
        if ( retpoline_safe() )
            thunk = THUNK_RETPOLINE;
        else
            ibrs = true;
    }

but it was changed by a request during review.  Sadly, the result is buggy as
it breaks the later fallback logic by allowing IBRS to appear as available
when in fact it isn't.

This in practice means that on repoline-unsafe hardware without IBRS, we
select THUNK_JUMP despite intending to select THUNK_RETPOLINE.

Reported-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>