virtio-net: fix leaking of ctx array

Fixes: commit d45b897b ("virtio_net: allow specifying context for rx")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>