ipv6: Fix dangling pointer when ipv6 fragment

ipv6: Fix dangling pointer when ipv6 fragment

At the beginning of ip6_fragment func, the prevhdr pointer is
obtained in the ip6_find_1stfragopt func.
However, all the pointers pointing into skb header may change
when calling skb_checksum_help func with
skb->ip_summed = CHECKSUM_PARTIAL condition.
The prevhdr pointe will be dangling if it is not reloaded after
calling __skb_linearize func in skb_checksum_help func.

Here, I add a variable, nexthdr_offset, to evaluate the offset,
which does not changes even after calling __skb_linearize func.

Fixes: 405c92f7a541 ("ipv6: add defensive check for CHECKSUM_PARTIAL skbs in ip_fragment")
Signed-off-by: Junwei Hu <[email protected]>
Reported-by: Wenhao Zhang <[email protected]>
Reported-by: [email protected]
Reviewed-by: Zhiqiang Liu <[email protected]>
Acked-by: Martin KaFai Lau <[email protected]>
Signed-off-by: David S. Miller <[email protected]>