pkg/cri/server: optimizations in unmountRecursive()
pkg/cri/server: optimizations in unmountRecursive()

Use a PrefixFilter() to get only the mounts we're interested in,
which removes the need to manually filter mounts from the mountinfo
results.

Additional optimizations can be made, as:

> ... there's a little known fact that `umount(MNT_DETACH)` is actually
> recursive in Linux, IOW this function can be replaced with
> `unix.Umount(target, unix.MNT_DETACH)` (or `mount.UnmountAll(target, unix.MNT_DETACH)`
>  (provided that target itself is a mount point).

https://github.com/containerd/containerd/pull/4578/commits/e8fb2c392f52ca8084fc88181599bd6c30650bba#r535450446

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

estesp/containerd,containerd/containerd,thaJeztah/containerd,mikebrow/containerd,estesp/containerd,mikebrow/containerd,thaJeztah/containerd,vdemeester/containerd,vdemeester/containerd,dmcgowan/containerd,containerd/containerd,dmcgowan/containerd