In case an XDP program is attached, reserve XDP_PACKET_HEADROOM
bytes at the beginning of the packet for the program to play
with.

Modify the XDP logic in the driver to fill-in the missing bits
and re-calculate offsets and length after the program has finished
running to properly reflect the current status of the packet.

We can then go and remove the limitation of not supporting XDP programs
where xdp_adjust_head is set.
Signed-off-by: Yuval Mintz <Yuval.Mintz@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>