autofs: Add FD_TYPES__AUTOFS_PIPE type

Add a fake fd type for autofs. This allows functions
like find_file_desc() work as expected, without
having two different file_desc with the same type
and same id.

Also, later, it will allow to delete autofs_create_fle()
and to use generic helper.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>