Add support for forwarding via XDP. Once the eBPF is attached,
driver would allocate & configure a designated transmission queue
meant solely for forwarding packets. Said queue would share the
receive-queue's interrupt line, and would have it's own Tx statistics.

Infrastructure changes required for this [spread-out through the code]:
 - Determine the DMA direction of the receive buffers based on the presence
of the eBPF program.
 - Turn the sw Tx ring into a union, as regular/XDP queues have different
needs for releasing resources after completion [regular requires the SKB,
XDP requires the transmitted page].
Signed-off-by: Yuval Mintz <Yuval.Mintz@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>