IPv6 sit fixes from upstream
kernel.org commits
36ca34cc3b8335eb1fe8bd9a1d0a2592980c3f02 sit: Add missing kfree_skb() on pskb_may_pull() failure
8db99e5717507729a9eb49108facb7e528340376 sit: Fail to create tunnel, if it already exists
8945a808f7d5efd21fa9fb6055d2dd7887bdd9d8 sit: Allow ip fragmentation when using nopmtudisc to fix package loss
292f4f3ce4b57f17a667cb34c72bca081dcc0281 sit: Clean up DF code by copying from IPIP
6dcdd1b3694a4fa2b85167a9c860c7613a7553c7 net/ipv6/sit.c: return unhandled skb to tunnel4_rcv