cgroup: fix inverted condition

A bug was introduced in acb14d3 "cgroup: when getting cgroup empty
notifications, always search up the tree".

When the given cgroup is found the hashmap, we should be happy and
return it, not go looking up the tree for another one.

Fixes the hanging NetworkManager on shutdown for me.

src/cgroup.c

@@ -364,7 +364,7 @@ int cgroup_bonding_get(Manager *m, const char *cgroup, CGroupBonding **bonding)
         assert(bonding);
 
         b = hashmap_get(m->cgroup_bondings, cgroup);
-        if (!b) {
+        if (b) {
                 *bonding = b;
                 return 1;
         }