pstree: allocate restore data as a tail of pstree_item (v2)

v2: Synchronize the argument type of __alloc_pstree_item and the
values you put into it. I.e. int-int or bool-bool, not bool-int.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>