ipv6: Fix protocol resubmission

UDP encapsulation is broken on IPv6. This is because the logic to resubmit
the nexthdr is inverted, checking for a ret value > 0 instead of < 0. Also,
the resubmit label is in the wrong position since we already get the
nexthdr value when performing decapsulation. In addition the skb pull is no
longer necessary either.

This changes the return value check to look for < 0, using it for the
nexthdr on the next iteration, and moves the resubmit label to the proper
location.

With these changes the v6 code now matches what we do in the v4 ip input
code wrt resubmitting when decapsulating.

Signed-off-by: Josh Hunt <johunt@akamai.com>
Acked-by: "Tom Herbert" <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>