net: packet: Introduce PACKET_XMIT_RECEIVE socket optionpacket-loop-back

The PACKET_XMIT_RECEIVE 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
dev_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_XMIT_RECEIVE, &one, sizeof(one));

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

Joint work with Daniel Borkmann.

Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>