af_packet: Use virtio_net_hdr_from_skb() directly.

Remove static function __packet_rcv_vnet(), which only called
virtio_net_hdr_from_skb() and BUG()ged out if an error code was
returned.  Instead, call virtio_net_hdr_from_skb() from the former
call sites of __packet_rcv_vnet() and actually use the error handling
code that is already there.

Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>