__netif_receive_skb_core() does a depressingly large amount of per-packet
 work that can't easily be listified, because the another_round looping
 makes it nontrivial to slice up into smaller functions.
Fortunately, most of that work disappears in the fast path:
 * Hardware devices generally don't have an rx_handler
 * Unless you're tcpdumping or something, there is usually only one ptype
 * VLAN processing comes before the protocol ptype lookup, so doesn't force
   a pt_prev deliver
 so normally, __netif_receive_skb_core() will run straight through and pass
 back the one ptype found in ptype_base[hash of skb->protocol].
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>