mnt: Create roots in criu context

In case root task restore failure we'll have to remove the
roots yard dir from criu, so we have to create one by
criu to at least have the dit name.

It's OK to do it in criu, since the yards is created in
the opts.root which is the same for any mnt ns we deal
with on restore.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>