mount: link dependent mounts (v3)

All shared mounts from one group are connected to circular list.
All slave are added into the proper master list.

v2: change variable name and fix a bug about adding shared mounts in a
circular list.
v3: handle errors of collect_shared

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>