ipv4: stop route notification sizing when nexthop group shrinks

In the Linux kernel, the following vulnerability has been resolved: ipv4: stop route notification sizing when nexthop group shrinks Commit 680aea08e78c ("net: ipv4: Emit notification when fib hardware flags are changed") added an RCU-only fib_nlmsg_size() call for asynchronous hardware flag notifications. fib_nlmsg_size() checks fib_info_num_path() before each iteration, then fib_info_nhc() independently reloads nh->nh_grp. If RTM_NEWNEXTHOP replaces a group with fewer paths between those loads, fib_info_nhc() returns NULL and fib_nexthop_nlmsg_size() dereferences it. Stop sizing when the indexed path is absent. Nexthop groups are dense, so the current snapshot has no later path.

Where it's fixed