core: when looking for the unit for a process, look at the PID hashmaps first

It's cheaper that going to cgroupfs, and also usually the better choice
since it's not racy and can map PIDs even if they were moved to a
different unit.

src/core/cgroup.c

@@ -1005,6 +1005,7 @@ Unit* manager_get_unit_by_cgroup(Manager *m, const char *cgroup) {
 
 Unit *manager_get_unit_by_pid(Manager *m, pid_t pid) {
         _cleanup_free_ char *cgroup = NULL;
+        Unit *u;
         int r;
 
         assert(m);
@@ -1012,6 +1013,14 @@ Unit *manager_get_unit_by_pid(Manager *m, pid_t pid) {
         if (pid <= 1)
                 return NULL;
 
+        u = hashmap_get(m->watch_pids1, LONG_TO_PTR(pid));
+        if (u)
+                return u;
+
+        u = hashmap_get(m->watch_pids2, LONG_TO_PTR(pid));
+        if (u)
+                return u;
+
         r = cg_pid_get_path(SYSTEMD_CGROUP_CONTROLLER, pid, &cgroup);
         if (r < 0)
                 return NULL;