riscv/barrier: Define __smp_{mb,rmb,wmb}

Introduce __smp_{mb,rmb,wmb}, and rely on the generic definitions
for smp_{mb,rmb,wmb}. A first consequence is that smp_{mb,rmb,wmb}
map to a compiler barrier on !SMP (while their definition remains
unchanged on SMP). As a further consequence, smp_load_acquire and
smp_store_release have "fence rw,rw" instead of "fence iorw,iorw".
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

arch/riscv/include/asm/barrier.h

@@ -34,9 +34,9 @@
 #define wmb()		RISCV_FENCE(ow,ow)
 
 /* These barriers do not need to enforce ordering on devices, just memory. */
-#define smp_mb()	RISCV_FENCE(rw,rw)
-#define smp_rmb()	RISCV_FENCE(r,r)
-#define smp_wmb()	RISCV_FENCE(w,w)
+#define __smp_mb()	RISCV_FENCE(rw,rw)
+#define __smp_rmb()	RISCV_FENCE(r,r)
+#define __smp_wmb()	RISCV_FENCE(w,w)
 
 /*
  * This is a very specific barrier: it's currently only used in two places in