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>

arch/arm/kernel/entry-armv.S

@@ -957,9 +957,7 @@ kuser_cmpxchg_fixup:
 
 #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]