core: make use of manager_loop()'s return value

The objective is returned in the return value, let's make use of that,
instead of reaching into the object.

src/core/main.c

@@ -1673,7 +1673,7 @@ static int invoke_main_loop(
                         return log_emergency_errno(r, "Failed to run main loop: %m");
                 }
 
-                switch (m->objective) {
+                switch ((ManagerObjective) r) {
 
                 case MANAGER_RELOAD: {
                         LogTarget saved_log_target;