mnt: restore shared mounts in a correct shared group

Currently when we have a shared group, we set a source mount for all
of them and then we need to update a source mount when we mount a first
mount from a shared group, because a shared id can be only inherited.

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