tools/virtio: use {READ,WRITE}_ONCE() in uaccess.h

As a step towards killing off ACCESS_ONCE, use {READ,WRITE}_ONCE() for the
virtio tools uaccess primitives, pulling these in from <linux/compiler.h>.

With this done, we can kill off the now-unused ACCESS_ONCE() definition.

Signed-off-by: Mark Rutland <[e-mail]>
Cc: Jason Wang <[e-mail]>
Cc: Michael S. Tsirkin <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Michael S. Tsirkin <[e-mail]>
Reviewed-by: Cornelia Huck <[e-mail]>
Reviewed-by: Jason Wang <[e-mail]>

tools/virtio/linux/uaccess.h

···
 #ifndef UACCESS_H
 #define UACCESS_H
-extern void *__user_addr_min, *__user_addr_max;
 
-#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
+#include <linux/compiler.h>
+
+extern void *__user_addr_min, *__user_addr_max;
 
 static inline void __chk_user_ptr(const volatile void *p, size_t size)
 {
···
 ({								\
 	typeof(ptr) __pu_ptr = (ptr);				\
 	__chk_user_ptr(__pu_ptr, sizeof(*__pu_ptr));		\
-	ACCESS_ONCE(*(__pu_ptr)) = x;				\
+	WRITE_ONCE(*(__pu_ptr), x);				\
 	0;							\
 })
 
···
 ({								\
 	typeof(ptr) __pu_ptr = (ptr);				\
 	__chk_user_ptr(__pu_ptr, sizeof(*__pu_ptr));		\
-	x = ACCESS_ONCE(*(__pu_ptr));				\
+	x = READ_ONCE(*(__pu_ptr));				\
 	0;							\
 })