chroot: let root be cleaned up by kernel

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
(cherry picked from commit 70dfea63ba2a8a6d8b367420039aeb6a1759c240)
Signed-off-by: Victor Vieux <vieux@docker.com>

pkg/chrootarchive/chroot_linux.go

@@ -52,13 +52,6 @@ func chroot(path string) (err error) {
 				err = errCleanup
 			}
 		}
-
-		if errCleanup := syscall.Unmount("/", syscall.MNT_DETACH); errCleanup != nil {
-			if err == nil {
-				err = fmt.Errorf("error unmounting root: %v", errCleanup)
-			}
-			return
-		}
 	}()
 
 	if err := syscall.PivotRoot(path, pivotDir); err != nil {