Fix O(n^2) restore in terms of the number of fds.

Since the processes and fds are restored from images in the increasing
order, and they are stored in sorted lists, CRIU always needed (at least
in my experiments) to traverse the whole list before inserting them.
With this change, lists are traversed in reverse, so the fds should be
inserted immediately.

Acked-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Radoslaw Burny <rburny@google.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>