zdtm/tempfs_subns: sync children with the main process

All static tests has to stop any activity before C/R.

./tempfs_subns --pidfile=tempfs_subns.pid --outfile=tempfs_subns.out --dirname=tempfs_subns.test
Run criu dump
Unable to kill 128: [Errno 3] No such process
Run criu restore
7: Old mounts lost: []
7: New mounts appeared: [('/rootfs/criu/test', '/'), ('/', '/proc'), ('/', '/dev/pts')]
:

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