snapshot creation: fsfreeze mountpoints, if needed

snapshot creation: fsfreeze mountpoints, if needed

fixes #2991, #2528.

creating a snapshot with rbd, after the syncfs finished successfully does not
guarantee that the snapshot has the state of the filesystem after syncfs.

suggestion taken from #2528 (running fsfreeze -f/-u before snapshotting on
the mountpoints)

added helper PVE::Storage::volume_snapshot_needs_fsfreeze, to indicate
which volumes need to be frozen/thawed. (and mocked it in the tests here).

Added the freeze to sync_container_namespace, since it needs to run inside the
container's mount namespace.

unfreezing happens in a sub of its own.

tests in #2991 seem to indicate that this helps to successfully create backups.

needs a versioned dependency bump on pve-storage

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>