ipv6: GRO should be ECN friendly

IPv4 side of the problem was addressed in commit a9e050f4
(net: tcp: GRO should be ECN friendly)

This patch does the same, but for IPv6 : A Traffic Class mismatch
doesnt mean flows are different, but instead should force a flush
of previous packets.

This patch removes artificial packet reordering problem.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>