s390: use generic memory barriers

The s390 kernel is SMP to 99.99%, we just didn't bother with a
non-smp variant for the memory-barriers. If the generic header
is used we'd get the non-smp version for free. It will save a
small amount of text space for CONFIG_SMP=n.
Suggested-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

arch/s390/include/asm/barrier.h

@@ -29,9 +29,6 @@
 #define __smp_mb()			mb()
 #define __smp_rmb()			rmb()
 #define __smp_wmb()			wmb()
-#define smp_mb()			__smp_mb()
-#define smp_rmb()			__smp_rmb()
-#define smp_wmb()			__smp_wmb()
 
 #define __smp_store_release(p, v)					\
 do {									\