LinuxKPI: add nexthdr definitions for IPv6

Add the nexthdr definitions for IPv6 which are used by wireless
drivers and were previously placed in an 80211 header file by
accident.

Obtained from: bz_iwlwifi
Sponsored by: The FreeBSD Foundation
Reviewed by: hselasky
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D31321