Sort dirs to unmount by pathlength in descending order
Sort dirs to unmount by pathlength in descending order

[#161177249](https://www.pivotaltracker.com/story/show/161177249)

Signed-off-by: Edwin Xie <6bc24e9c160281972837d9e13287696d26612564@pivotal.io>

cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release