mount: make open_mountpoint handle overmouts properly

dump of VZ7 ct fails, if we have overmounted tmpfs inside:

[root@silo ~]# prlctl enter su-test-2
entered into CT
CT-829e7b28 /# mkdir /mnt/overmntedtmp
CT-829e7b28 /# mount -t tmpfs tmpfs /mnt/overmntedtmp/
CT-829e7b28 /# mount -t tmpfs tmpfs /mnt
CT-829e7b28 /# logout

[root@silo ~]# prlctl suspend su-test-2
Suspending the CT...
Failed to suspend the CT: PRL_ERR_VZCTL_OPERATION_FAILED (Details: Will skip in-flight TCP connections
(01.657913) Error (criu/mount.c:1202): mnt: Can't open ./mnt/overmntedtmp: No such file or directory
(01.662528) Error (criu/util.c:709): exited, status=1
(01.664329) Error (criu/util.c:709): exited, status=1
(01.664694) Error (criu/cr-dump.c:2005): Dumping FAILED.
Failed to checkpoint the Container
All dump files and logs were saved to /vz/private/829e7b28-f204-4bce-b09f-d203b99befd4/dump/Dump.fail
Checkpointing failed
)

Criu wants to dump the contents of /mnt/overmntedtmp/ mount but it is
unavailable. So we copy the mount namespace in such a case and unmount
overmounts to access what we want to dump.

Actual usecase here is dumping CT with active mariadb and ssh
connection. Together they happen to create such overmount. As by default
systemd creates a separate mount namespace for mysql and also mounts
tmpfs to /run/user in it, and when ssh(root) is connected - systemd also
mounts tmpfs in container root mount namespace to /run/user/0 for user
files. As /run is slave mount /run/user/0 also propagates to mysql's
mount namespace and initially becomes overmounted by /run/user.

https://jira.sw.ru/browse/PSBM-57362



remove __maybe_unused for mnt_is_overmounted and umount_overmounts

changes in v2:
1) Use clone not fork, share resources with parent same as in
call_in_child_process.
2) Do not enter userns (create helper) for non-overmounted mounts. Thus
return back setns/resorens logic.
3) Helper opens fd for parent directly due to CLONE_FILES, remove futex.
4) Check helper exit status properly.
5) Add get_clean_fd helper.
6) Add better comments.

changes in v3:
1) Pass fd from helper through args instead of ret code, fix ret code
checking.
2) Add \n to pr_err in open_mountpoint

changes in v5:
Make comments even better.

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