net: Fix a bug when handling IPv6 packets
net: Fix a bug when handling IPv6 packets

When gem5 runs with the networking support, it will run into an assertion
fail and aborted. This is because it tries to calculate checksum for IPv6
packet and this makes the IpPtr pointer ``ip'' become NULL. For that there
is functions and classes for IPv6 in base/inet.cc, I added IPv6 support for
i8254xGBe.cc. Because IPv6 header does not have identification number,  I
ignored some of the debug messages using ip->id().

Change-Id: Ida5e36aefd2c5c26053f8152a0aac24191e7757c
Reviewed-on: https://gem5-review.googlesource.com/12339
Reviewed-by: Earl Ou <82591a79653dd29c9ce06ea2b578447596bef9ab@google.com>
Reviewed-by: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>

TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5