LP#1465998 -- overlayfs "rmdir" panic issue

[Andy Whitcroft]

The overlayfs emergency CVE fix backports for trusty, precise and utopic
have the potential to trigger a kernel panic in the face of cirtain
operations against directory objects only on the lower layer (at least).
Following this email are patches for trusty, precise and utopic.

Proposing for SRU to trusty, precise and utopic.  Vivid and Wily should
not be affected.

Reviews appreciated.

-apw