CVE-2026-64538 ipv6: Fix null-ptr-deref in fib6_nh_mtu_change().

In the Linux kernel, the following vulnerability has been resolved: ipv6: Fix null-ptr-deref in fib6nhmtuchange. fib6nhmtuchange re-fetches idev via in6devgetarg-dev and dereferences idev-cnf.mtu6 without a NULL check. addrconfifdown clears dev-ip6ptr with RCUINITPOINTER after rt6disableip has...