x86/entry/64: Ensure %ebx handling correct in xen_failsafe_callback

x86/entry/64: Ensure %ebx handling correct in xen_failsafe_callback

error_entry and error_exit communicate the user vs. kernel status of
the frame using %ebx. Make sure %ebx is set properly in
xen_failsafe_callback().

This fix is different from the one provided in the XSA to minimize
risk of a regression. This alternative fix was suggested
by Andy Lutomirski in the original patch (Commit b3681dd548d0
("x86/entry/64: Remove %ebx handling from error_entry/exit"))

Orabug: 28402927
CVE: CVE-2018-14678

This is XSA-274

Signed-off-by: George Kennedy <george.kennedy@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>