Add write barriers

And enable them in solarisaio, then it should be safe to use the
signal driven IO "interrupts".

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/arch/arch-s390.h b/arch/arch-s390.h
index c4ec1d1..dfb7899 100644
--- a/arch/arch-s390.h
+++ b/arch/arch-s390.h
@@ -14,5 +14,6 @@
 
 #define nop		asm volatile ("diag 0,0,68" : : : "memory")
 #define read_barrier()	asm volatile("bcr 15,0" : : : "memory")
+#define write_barrier()	asm volatile("bcr 15,0" : : : "memory")
 
 #endif