net: Add __dev_forward_skb

This patch adds the helper __dev_forward_skb which is identical to
dev_forward_skb except that it doesn't actually inject the skb into
the stack.  This is useful where we wish to have finer control over
how the packet is injected, e.g., via netif_rx_ni or netif_receive_skb.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>