igb: Correctly get protocol information

We can't look at the socket to get protocol information. We should
instead look directly at the packet, and hope there are no IPv6
option headers.

Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>