core: mount namespaces: Remove auxiliary bind mounts directory after unit termination

Unit that requires its own mount namespace creates a temporary directory
to implement dynamic bind mounts (org.freedesktop.systemd1.Manager.BindMountUnit).
However, this directory is never removed and they will accumulate for
each unique unit (e.g. templated units of systemd-coredump@).

Attach the auxiliary runtime directory existence to lifetime of other
"runtime" only per-unit directories.

src/core/unit.c

@@ -5765,6 +5765,7 @@ void unit_destroy_runtime_data(Unit *u, const ExecContext *context) {
                 exec_context_destroy_runtime_directory(context, u->manager->prefix[EXEC_DIRECTORY_RUNTIME]);
 
         exec_context_destroy_credentials(context, u->manager->prefix[EXEC_DIRECTORY_RUNTIME], u->id);
+        exec_context_destroy_mount_ns_dir(u);
 }
 
 int unit_clean(Unit *u, ExecCleanMask mask) {