CVE-2026-74561 nexthop: avoid unlocked f6i_list walk in nh_rt_cache_flush

In the Linux kernel, the following vulnerability has been resolved: nexthop: avoid unlocked f6ilist walk in nhrtcacheflush nhrtcacheflush walks nh-f6ilist during an RTNL-serialized nexthop replace without holding nh-lock, racing the unlocked IPv6 route add/delete that mutate the list under nh-loc...