CVE-2026-99073: ipv4: stop exception dump when nexthop group shrinks

In the Linux kernel, the following vulnerability has been resolved:

ipv4: stop exception dump when nexthop group shrinks

Commit 4ce5dc9316de ("inet: switch inetdumpfib() to RCU protection") allowed IPv4 FIB dumps to run concurrently with nexthop replacement.

fibdumpinfofnhe() uses fibinfonumpath() as the loop bound, then fibinfonhc() independently reloads nh->nhgrp. If RTMNEWNEXTHOP replaces a group with fewer paths between those loads, fibinfonhc() returns NULL and the dump dereferences it through nhcflags.

Stop the walk when the indexed path is absent. Nexthop groups are dense, so the current snapshot has no later path.