CVE-2026-63924 ipv6: exthdrs: refresh nh pointer after ipv6_hop_jumbo()

In the Linux kernel, the following vulnerability has been resolved: ipv6: exthdrs: refresh nh pointer after ipv6hopjumbo ipv6hopjumbo calls pskbtrimrcsum, which can change skb pointers. Let's recompute nh pointer to make sure any change won't mess things up...