net: print proper warning on dst underflow

Proper warnings with stack traces make it much easier to figure out
what's doing the double free and create more meaningful bug reports from
users.
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dst.c

@@ -172,7 +172,7 @@ void dst_release(struct dst_entry *dst)
 		int newrefcnt;
 
 		newrefcnt = atomic_dec_return(&dst->__refcnt);
-		if (unlikely(newrefcnt < 0))
+		if (WARN_ONCE(newrefcnt < 0, "dst_release underflow"))
 			net_warn_ratelimited("%s: dst:%p refcnt:%d\n",
 					     __func__, dst, newrefcnt);
 		if (!newrefcnt)
@@ -187,7 +187,7 @@ void dst_release_immediate(struct dst_entry *dst)
 		int newrefcnt;
 
 		newrefcnt = atomic_dec_return(&dst->__refcnt);
-		if (unlikely(newrefcnt < 0))
+		if (WARN_ONCE(newrefcnt < 0, "dst_release_immediate underflow"))
 			net_warn_ratelimited("%s: dst:%p refcnt:%d\n",
 					     __func__, dst, newrefcnt);
 		if (!newrefcnt)