net: ratelimit warnings about dst entry refcount underflow or overflow

Kernel generates a lot of warnings when dst entry reference counter overflows and becomes negative. That bug was seen several times at machines with outdated 3.10.y kernels. Most like it's already fixed in upstream. Anyway that flood completely kills machine and makes further debugging impossible.

---
 net/core/dst.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

index e956ce6d1378..002144bea935 100644
@@ -284,7 +284,9 @@ void dst_release(struct dst_entry *dst)
 		int newrefcnt;
 
 		newrefcnt = atomic_dec_return(&dst->__refcnt);
-		WARN_ON(newrefcnt < 0);
+		if (unlikely(newrefcnt < 0))
+			net_warn_ratelimited("%s: dst:%p refcnt:%d\n",
+					     __func__, dst, newrefcnt);
 		if (unlikely(dst->flags & DST_NOCACHE) && !newrefcnt)
 			call_rcu(&dst->rcu_head, dst_destroy_rcu);
 	}

From: Konstantin Khlebnikov <[email protected]>
Date: Fri, 17 Jul 2015 14:01:11 +0300

Applied.