xen: arm: Use dmb for smp barriers

The full power of dsb is not required in this context.

Also change wmb() to be dsb() directly instead of indirectly via mb(), for
clarity.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Tim Deegan <tim@xen.org>

xen/include/asm-arm/system.h

@@ -18,11 +18,11 @@
 
 #define mb()            dsb()
 #define rmb()           dsb()
-#define wmb()           mb()
+#define wmb()           dsb()
 
-#define smp_mb()        mb()
-#define smp_rmb()       rmb()
-#define smp_wmb()       wmb()
+#define smp_mb()        dmb()
+#define smp_rmb()       dmb()
+#define smp_wmb()       dmb()
 
 #define xchg(ptr,x) \
         ((__typeof__(*(ptr)))__xchg((unsigned long)(x),(ptr),sizeof(*(ptr))))