RISC-V: Add READ_ONCE in arch_spin_is_locked()

This was just incorrect in the original version.
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

arch/riscv/include/asm/spinlock.h

@@ -24,7 +24,7 @@
 
 /* FIXME: Replace this with a ticket lock, like MIPS. */
 
-#define arch_spin_is_locked(x)	((x)->lock != 0)
+#define arch_spin_is_locked(x)	(READ_ONCE((x)->lock) != 0)
 
 static inline void arch_spin_unlock(arch_spinlock_t *lock)
 {