atomics: Remove redundant barrier()'s

Remove the redundant barrier() after the fence as agreed in previous
discussion here:
https://lists.gnu.org/archive/html/qemu-devel/2016-04/msg00489.htmlSigned-off-by: NPranith Kumar <bobby.prani@gmail.com>
Message-Id: <20160824204424.14041-3-bobby.prani@gmail.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

include/qemu/atomic.h

@@ -72,16 +72,16 @@
  * Add one here, and similarly in smp_rmb() and smp_read_barrier_depends().
  */
 
-#define smp_mb()    ({ barrier(); __atomic_thread_fence(__ATOMIC_SEQ_CST); barrier(); })
-#define smp_wmb()   ({ barrier(); __atomic_thread_fence(__ATOMIC_RELEASE); barrier(); })
-#define smp_rmb()   ({ barrier(); __atomic_thread_fence(__ATOMIC_ACQUIRE); barrier(); })
+#define smp_mb()    ({ barrier(); __atomic_thread_fence(__ATOMIC_SEQ_CST); })
+#define smp_wmb()   ({ barrier(); __atomic_thread_fence(__ATOMIC_RELEASE); })
+#define smp_rmb()   ({ barrier(); __atomic_thread_fence(__ATOMIC_ACQUIRE); })
 
 /* Most compilers currently treat consume and acquire the same, but really
  * no processors except Alpha need a barrier here.  Leave it in if
  * using Thread Sanitizer to avoid warnings, otherwise optimize it away.
  */
 #if defined(__SANITIZE_THREAD__)
-#define smp_read_barrier_depends() ({ barrier(); __atomic_thread_fence(__ATOMIC_CONSUME); barrier(); })
+#define smp_read_barrier_depends() ({ barrier(); __atomic_thread_fence(__ATOMIC_CONSUME); })
 #elsif defined(__alpha__)
 #define smp_read_barrier_depends()   asm volatile("mb":::"memory")
 #else