close_olds_fds() knows nothing about more than one set of service file
descriptros, so it's better to call it before forking children as it was
bedore 9d60724e ("restore: restore mntns before creating private vma-s")

The root task restores all processes and pin them with file descriptors,
then a task restores a mount namespace by opening the file descriptor of
the root task via /proc/pid/fd/X.

Reported-by: Mr Jenkins
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>