Fedora bind-mounts a part of the root mount to itself. Currently we
don't allow to mount children of a shared mount, if other mount from
this shared group are not mounted.

This patch adds an exclusion for cases, when a child has the same
group. We allow to mount a child, if wider mounts are mounted.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>