virtio: net: Add missing virtqueue_kick in free_pkt

virtio: net: Add missing virtqueue_kick in free_pkt

Every virtqueue_add must eventually be followed by virtqueue_kick for to
properly notify the peer that new buffers have been put into the queue.

This is currently missing for virtio-net and may result in non-working
network when the host has depleted the rx buffers and waits for new
buffers. Depending on the host it may busy poll on the virtio queue or
wait for the kick. Qemu does the latter and may break.

Signed-off-by: Christian Speich <c.speich@avm.de>