AUTOFS: allow removing entries from hash table
    
    There is a timed desctructor in the autofs responder that, when the
    entry timeout passes, removes the autofs map from the hash table while
    the map is freed. This patch adds a hash delete callback so that if the
    map is removed from the hash table with hash_delete, its hash table
    pointer will be invalidated. Later, when the entry is being freed, the
    destructor won't attempt to remove it from the hash table.