mnt: remember to restore MS_UNBINDABLE

Note that if the root is unbindable then restore will fail because
cr_pivot_root() tries to bind mount the put dir. If this is a case we want to
support, we may want to rearrange how this code is called.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>