CVE-2026-63920 ipv6: validate extension header length before copying to cmsg

In the Linux kernel, the following vulnerability has been resolved: ipv6: validate extension header length before copying to cmsg ip6datagramrecvspecificctl builds IPV6HOPOPTS,DSTOPTS,RTHDR cmsgs and their IPV62292 legacy counterparts by trusting the on-wire hdrlen byte ptr1 when computing the...