use SMP barrier in common code dealing with shared memory protocols

use SMP barrier in common code dealing with shared memory protocols

Xen currently makes no strong distinction between the SMP barriers (smp_mb
etc) and the regular barrier (mb etc). In Linux, where we inherited these
names from having imported Linux code which uses them, the SMP barriers are
intended to be sufficient for implementing shared-memory protocols between
processors in an SMP system while the standard barriers are useful for MMIO
etc.

On x86 with the stronger ordering model there is not much practical difference
here but ARM has weaker barriers available which are suitable for use as SMP
barriers.

Therefore ensure that common code uses the SMP barriers when that is all which
is required.

On both ARM and x86 both types of barrier are currently identical so there is
no actual change. A future patch will change smp_mb to a weaker barrier on
ARM.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>