igb(4): Update and fix HW errata

• HW errata workaround for IPv6 offload w/ extension headers
• Edited start of if_igb.c (Device IDs / #includes) to match ixgbe/ixl

Differential Revision: https://reviews.freebsd.org/D3165
Submitted by: erj
MFC after: 1 month
Sponsored by: Intel Corporation