restore: Do not iterate over parent's files to find leftovers

This patch speeds up creation of child process by disabling
iteration over open files for the most cases. Really, we don't
need that now, as previous patches make parent files do not leak:

mnt namespace fds are stored in fdstore, pid proc files
are closed directly.

So, now we can skip closing old files for the most cases,
except some CLONE_FILES cases: we need that only if parent
have CLONE_FILES in its flags (and for root_item).

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