There is a scenario when pstree_fd may be tried
to close several times -- if we start crtools
with "detach" option.

So simply make restore_root_task to close opened
file descriptor, this also simplifies the code.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>