mount: wait when mntns will be created to get its root (v2)

v2: add comments and rename ns_created to ns_populated.

Reported-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>