virtio-net: don't copy a virtio_net_hdr in receiver()

don't copy a virtio virtio_net_hdr to a local buffer in receiver().
This prevents CPU's optimizations.
Cast the mbuf's beginning to the virtio_net_hdr and use the relevant
fields where needed allowing a proper load prediction (as a result of
branch prediction).

Reviewed-by: Dor Laor <dor@cloudius-systems.com>
Reviewed-by: Zhi Yong Wu <zwu.kernel@gmail.com>
Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>