[S390] atomic: use ACCESS_ONCE() for atomic_read()

Let's make atomic_read() and atomic_set() behave like on all/most other
architectures. Generated code is identical with gcc 4.5.2.
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/atomic.h

@@ -36,14 +36,12 @@
 
 static inline int atomic_read(const atomic_t *v)
 {
-	barrier();
-	return v->counter;
+	return ACCESS_ONCE(v->counter);
 }
 
 static inline void atomic_set(atomic_t *v, int i)
 {
 	v->counter = i;
-	barrier();
 }
 
 static inline int atomic_add_return(int i, atomic_t *v)
@@ -128,14 +126,12 @@ static inline int atomic_add_unless(atomic_t *v, int a, int u)
 
 static inline long long atomic64_read(const atomic64_t *v)
 {
-	barrier();
-	return v->counter;
+	return ACCESS_ONCE(v->counter);
 }
 
 static inline void atomic64_set(atomic64_t *v, long long i)
 {
 	v->counter = i;
-	barrier();
 }
 
 static inline long long atomic64_add_return(long long i, atomic64_t *v)