The root yard is used to clean up ghost files.

Now try_clean_remaps() is called from depopulate_roots_yard(), so
the code about switching mount namespaces was moved to
depopulate_roots_yard().

v2: call clean_remaps() when processes are restored in
    the host mount namespace.

Now depopulate_roots_yard() is called from the root task before
finishing CR_STATE_FORKING.

I moved it to the criu process and do it after clean_remaps(), because
clean_remaps() uses the roots yard.

It's called after openning all files, because only at this moment we can
be sure that all link remap files can be removed.

restore_task_with_children()		| restore_root_task()
-----------------------------------------------------------------------
depopulate_roots_yard()			|
restore_finish_stage(CR_STATE_FORKING)	|
prepare_fds()				|
open_vmas()				|
					| restore_switch_stage(CR_STATE_RESTORE_SIGCHLD)
					| clean_remaps = 0;

If something fails between CR_STATE_FORKING and CR_STATE_RESTORE_SIGCHLD,
try_clean_remaps will be called().

try_clean_remaps()
  try_clean_ghost()
    rst_get_mnt_root()
      print_ns_root()
	snprintf(buf, bs, "%s/%d", mnt_roots, ns->id);

it uses mnt_roots, actually it is what we called the roots yard.
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>