CVE-2026-74562 nexthop: take nh->lock for f6i_list walks in replace check and notify

In the Linux kernel, the following vulnerability has been resolved: nexthop: take nh-lock for f6ilist walks in replace check and notify fib6checknhlist and nexthopreplacenotify walk nh-f6ilist during an RTNL-serialized nexthop replace without holding nh-lock. IPv6 RTMNEWROUTE/RTMDELROUTE run...