virtio: add unlikely() to WARN_ON_ONCE()

The condition to test is unlikely() to be true. Add the hint.
Signed-off-by: Igor Stoppa <igor.stoppa@huawei.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Virtualization@lists.linux-foundation.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

tools/virtio/linux/kernel.h

@@ -127,7 +127,7 @@ static inline void free_page(unsigned long addr)
 #define dev_err(dev, format, ...) fprintf (stderr, format, ## __VA_ARGS__)
 #define dev_warn(dev, format, ...) fprintf (stderr, format, ## __VA_ARGS__)
 
-#define WARN_ON_ONCE(cond) ((cond) ? fprintf (stderr, "WARNING\n") : 0)
+#define WARN_ON_ONCE(cond) (unlikely(cond) ? fprintf (stderr, "WARNING\n") : 0)
 
 #define min(x, y) ({				\
 	typeof(x) _min1 = (x);			\