move_mount: allow to add a mount into an existing group

Previously a sharing group (shared and master ids pair) can be only
inherited when mount is created via bindmount. This patch adds an
ability to add an existing private mount into an existing sharing group.

With this functionality one can first create the desired mount tree from
only private mounts (without the need to care about undesired mount
propagation or mount creation order implied by sharing group
dependencies), and next then setup any desired mount sharing between
those mounts in tree as needed.

This allows CRIU to restore any set of mount namespaces, mount trees and
sharing group trees for a container.

We have many issues with restoring mounts in CRIU related to sharing
groups and propagation:
- reverse sharing groups vs mount tree order requires complex mounts
  reordering which mostly implies also using some temporary mounts
(please see https://lkml.org/lkml/2021/3/23/569 for more info)

- mount() syscall creates tons of mounts due to propagation
- mount re-parenting due to propagation
- "Mount Trap" due to propagation
- "Non Uniform" propagation, meaning that with different tricks with
  mount order and temporary children-"lock" mounts one can create mount
  trees which can't be restored without those tricks
(see https://www.linuxplumbersconf.org/event/7/contributions/640/)

With this new functionality we can resolve all the problems with
propagation at once.

Link: https://lore.kernel.org/r/[email protected]
Cc: Eric W. Biederman <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Mattias Nissler <[email protected]>
Cc: Aleksa Sarai <[email protected]>
Cc: Andrei Vagin <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: lkml <[email protected]>
Co-developed-by: Andrei Vagin <[email protected]>
Acked-by: Christian Brauner <[email protected]>
Signed-off-by: Pavel Tikhomirov <[email protected]>
Signed-off-by: Andrei Vagin <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

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