bhyve: vtnet: fix locking on receive

The vsc_rx_ready and the RX virtqueue is protected by the rx_mtx lock.
However, pci_vtnet_ping_rxq() (currently called only once after each
device reset) accesses those without acquiring the lock.

Reviewed by:	markj
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D20609

usr.sbin/bhyve/pci_virtio_net.c

@@ -583,10 +583,12 @@ pci_vtnet_ping_rxq(void *vsc, struct vqueue_info *vq)
 	/*
 	 * A qnotify means that the rx process can now begin
 	 */
+	pthread_mutex_lock(&sc->rx_mtx);
 	if (sc->vsc_rx_ready == 0) {
 		sc->vsc_rx_ready = 1;
 		vq_kick_disable(vq);
 	}
+	pthread_mutex_unlock(&sc->rx_mtx);
 }
 
 static void