restore: Do not allocate heads for post-prep actions

Now the post-prep actions are xmalloc()-ed then added to the list.
Let's make it sit right on the structures that need the post-prep,
this would make some things much simpler and nicer soon.

The approach is inspired by kernel rcu_head-s :)

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>