mount: allow overmount on parent with shared group

In CT, we do:

mkdir -p /a/b/c1
mkdir -p /c2
mount --bind /c2 /a/b/c1
mount --rbind /a/b /a

And after that container is not dumpable with error:

mnt: Unable to handle mounts under 146:./a

Just because overmounts with shared parent group are prohibited,
but I can't see any problem with enabling them.

https://jira.sw.ru/browse/PSBM-69501


Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>