core: cleanup unit's dropin directories from global cache

When user creates dropin files via API (e.g. systemctl set-property ...)
we put the dropin directory path into unit_path_cache. Drop those
directories from the cache in unit_free() and prevent memory leak.

Follow-up for fce94c5c56.

src/core/unit.c

@@ -677,6 +677,8 @@ static void unit_remove_transient(Unit *u) {
         if (!u->transient)
                 return;
 
+        const char *dropin_directory = strjoina(u->id, ".d");
+
         STRV_FOREACH(i, u->dropin_paths) {
                 _cleanup_free_ char *p = NULL, *pp = NULL;
 
@@ -690,6 +692,10 @@ static void unit_remove_transient(Unit *u) {
                 if (!path_equal(u->manager->lookup_paths.transient, pp))
                         continue;
 
+                /* Drop the transient drop-in directory also from unit path cache. */
+                if (path_equal(last_path_component(p), dropin_directory))
+                        free(set_remove(u->manager->unit_path_cache, p));
+
                 (void) unlink(*i);
                 (void) rmdir(p);
         }