bpf: add XDP_TX xdp_action for direct forwarding

XDP enabled drivers must transmit received packets back out on the same
port they were received on when a program returns this action.

Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>