sh: define __smp_xxx, fix smp_store_mb for !SMP

sh variant of smp_store_mb() calls xchg() on !SMP which is stronger than
implied by both the name and the documentation.

define __smp_store_mb instead: code in asm-generic/barrier.h
will then define smp_store_mb correctly depending on
CONFIG_SMP.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

arch/sh/include/asm/barrier.h

@@ -32,7 +32,8 @@
 #define ctrl_barrier()	__asm__ __volatile__ ("nop;nop;nop;nop;nop;nop;nop;nop")
 #endif
 
-#define smp_store_mb(var, value) do { (void)xchg(&var, value); } while (0)
+#define __smp_store_mb(var, value) do { (void)xchg(&var, value); } while (0)
+#define smp_store_mb(var, value) __smp_store_mb(var, value)
 
 #include <asm-generic/barrier.h>