vma: Mark cow roots

Next patch will stop premapping some private vmas. In particular -- those,
that are not COW-ed with anyone. To make this work we need to distinguish
vmas that are not cowed with anyone from those cowed with children only.
Currently both have vma->parent pointer set to NULL, so for former let's
introduce the special mark.

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