arm64: barriers: use barrier() instead of smp_mb() when !SMP

The recently introduced acquire/release accessors refer to smp_mb()
in the !CONFIG_SMP case. This is confusing when reading the code, so use
barrier() directly when we know we're UP.
Acked-by: NCatalin Marinas <catalin.marinas@arm.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/barrier.h

@@ -40,7 +40,7 @@
 #define smp_store_release(p, v)						\
 do {									\
 	compiletime_assert_atomic_type(*p);				\
-	smp_mb();							\
+	barrier();							\
 	ACCESS_ONCE(*p) = (v);						\
 } while (0)
 
@@ -48,7 +48,7 @@ do {									\
 ({									\
 	typeof(*p) ___p1 = ACCESS_ONCE(*p);				\
 	compiletime_assert_atomic_type(*p);				\
-	smp_mb();							\
+	barrier();							\
 	___p1;								\
 })