tools/virtio: stub might_sleep and synchronize_rcu

Add might_sleep() and synchronize_rcu() stubs needed by virtio_config.h.

might_sleep() is a no-op, synchronize_rcu doesn't work but we don't
need it to.

Created using Cursor CLI.

Message-ID: <[e-mail]>
Acked-by: Jason Wang <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>

tools/virtio/linux/kernel.h

···
 	return false;
 }
 
+#define might_sleep() do { } while (0)
+
+static inline void synchronize_rcu(void)
+{
+	assert(0);
+}
+
 #define min(x, y) ({				\
 	typeof(x) _min1 = (x);			\
 	typeof(y) _min2 = (y);			\