Use mount.Target to specify subdirectory of rootfs mount

- Add Target to mount.Mount.
- Add UnmountMounts to unmount a list of mounts in reverse order.
- Add UnmountRecursive to unmount deepest mount first for a given target, using
moby/sys/mountinfo.

Signed-off-by: Edgar Lee <edgarhinshunlee@gmail.com>

cmd/ctr/commands/snapshots/snapshots.go

@@ -22,6 +22,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"path/filepath"
 	"strings"
 	"text/tabwriter"
 	"time"
@@ -640,6 +641,6 @@ func printNode(name string, tree *snapshotTree, level int) {
 func printMounts(target string, mounts []mount.Mount) {
 	// FIXME: This is specific to Unix
 	for _, m := range mounts {
-		fmt.Printf("mount -t %s %s %s -o %s\n", m.Type, m.Source, target, strings.Join(m.Options, ","))
+		fmt.Printf("mount -t %s %s %s -o %s\n", m.Type, m.Source, filepath.Join(target, m.Target), strings.Join(m.Options, ","))
 	}
 }