xen/x86: Replace appropriate mandatory barriers with SMP barriers

xen/x86: Replace appropriate mandatory barriers with SMP barriers

There is no functional change.  Xen currently assignes smp_* meaning to
the non-smp_* barriers.

All of these uses are just to deal with shared memory between multiple
processors, which means that the smp_*() varients are the correct ones to use.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>