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.

Fixes: 6b2e04b ("net: allow user to set metric on default route learned via Router Advertisement")
Signed-off-by: Xin Xiong <xiongx18@fudan.edu.cn>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ndisc.c

@@ -1378,6 +1378,9 @@ static void ndisc_router_discovery(struct sk_buff *skb)
 	if (!rt && lifetime) {
 		ND_PRINTK(3, info, "RA: adding default router\n");
 
+		if (neigh)
+			neigh_release(neigh);
+
 		rt = rt6_add_dflt_router(net, &ipv6_hdr(skb)->saddr,
 					 skb->dev, pref, defrtr_usr_metric);
 		if (!rt) {