ARM: Fix wrong dmb

The __kuser_cmpxchg code uses an ARMv6 dmb instruction, rather than
one based upon the architecture being built for.  Switch to using
the macro provided for this purpose, which also eliminates the
need for an ifdef.

Acked-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
index d2903e3..6c5cf36 100644
--- a/arch/arm/kernel/entry-armv.S
+++ b/arch/arm/kernel/entry-armv.S
@@ -957,9 +957,7 @@
 
 #else
 
-#ifdef CONFIG_SMP
-	mcr	p15, 0, r0, c7, c10, 5	@ dmb
-#endif
+	smp_dmb
 1:	ldrex	r3, [r2]
 	subs	r3, r3, r0
 	strexeq	r3, r1, [r2]