net: fib_notifier: don't return positive values on fib registration

The function fib6_walk_continue() cannot return a positive value when
called from register_fib_notifier(), but ignoring causes static analyzer to
generate warnings in users of register_fib_notifier() that try to convert
returned error code to pointer with ERR_PTR(). Handle such case by
explicitly checking for positive error values and converting them to
-EINVAL in fib6_tables_dump().

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Suggested-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_fib.c

@@ -499,15 +499,16 @@ int fib6_tables_dump(struct net *net, struct notifier_block *nb,
 
 		hlist_for_each_entry_rcu(tb, head, tb6_hlist) {
 			err = fib6_table_dump(net, tb, w);
-			if (err < 0)
+			if (err)
 				goto out;
 		}
 	}
 
 out:
 	kfree(w);
 
-	return err;
+	/* The tree traversal function should never return a positive value. */
+	return err > 0 ? -EINVAL : err;
 }
 
 static int fib6_dump_node(struct fib6_walker *w)