Refactor mounts into pkg to make changes easier

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)

libcontainer/mount/pivotroot.go

@@ -0,0 +1,31 @@
+// +build linux
+
+package mount
+
+import (
+	"fmt"
+	"github.com/dotcloud/docker/pkg/system"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"syscall"
+)
+
+func PivotRoot(rootfs string) error {
+	pivotDir, err := ioutil.TempDir(rootfs, ".pivot_root")
+	if err != nil {
+		return fmt.Errorf("can't create pivot_root dir %s", pivotDir, err)
+	}
+	if err := system.Pivotroot(rootfs, pivotDir); err != nil {
+		return fmt.Errorf("pivot_root %s", err)
+	}
+	if err := system.Chdir("/"); err != nil {
+		return fmt.Errorf("chdir / %s", err)
+	}
+	// path to pivot dir now changed, update
+	pivotDir = filepath.Join("/", filepath.Base(pivotDir))
+	if err := system.Unmount(pivotDir, syscall.MNT_DETACH); err != nil {
+		return fmt.Errorf("unmount pivot_root dir %s", err)
+	}
+	return os.Remove(pivotDir)
+}