net: fix potential refcount leak in ndisc_router_discovery()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: net: fix potential refcount leak in ndisc_router_discovery() The issue happens on specific paths in the function. After both the object `rt` and `neigh` are grabbed successfully, when `lifetime` is nonzero but the metric needs change, the function just deletes the route and set `rt` to NULL. Then, it may try grabbing `rt` and `neigh` again if above conditions hold. The function simply overwrite `neigh` if succeeds or returns if fails, without decreasing the reference count of previous `neigh`. This may result in memory leaks. Fix it by decrementing the reference count of `neigh` in place.

References

• https://git.kernel.org/stable/c/ffb15594433391fd7885eb88ce5a7f7bdeefbb15
• https://git.kernel.org/stable/c/7998043d31d000c3a93f46182e6569dd0eecda34
• https://git.kernel.org/stable/c/7396ba87f1edf549284869451665c7c4e74ecd4f