x86/microcode: add SPEC_CTRL_SSBD to x86_spec_ctrl_mask on late loading.

x86/microcode: add SPEC_CTRL_SSBD to x86_spec_ctrl_mask on late loading.

This is required so that we don't filter out the SPEC_CTRL_SSBD bit from
what the guest is trying to write to the MSR_IA32_SPEC_CTRL in
x86_virt_spec_ctrl().  Failure to do would make it look like the guest
correctly enabled SSBD when it did not, as reading back the MSR from the
guest would not show the bit was filtered out, giving a false sense of
security.

Orabug: 29642139

Reported-by: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Signed-off-by: Mihai Carabas <mihai.carabas@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>