net: packet: Introduce PACKET_RX_PUMP_BACK socket optionpacket-rx-pump-back

The PACKET_RX_PUMP_BACK socket options allows to immediately pump
packets sent through PF_PACKET back into the network stack via
dev_forward_skb() instead of submitting them to the device driver using
deve_queue_xmit(). This feature might be useful to load the network
stack with a lot of packets, similar to pktgen. Enable the
setting using:

    int one = 1;
    setsockopt(fd, SOL_PACKET, PACKET_RX_PUMP_BACK, &one, sizeof(one));

By default, PACKET_RX_PUMP_BACK is disabled and needs to be explicitly enabled
on PF_PACKET sockets.

Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>