Install the Linux kernel from Debian Jessie (Will-fix: #9341).

Now that Linux 3.16.7-ckt9-3 has migrated to Debian Jessie (well,
jessie/updates), let's install the kernel from there instead of Debian
testing, so we won't get Linux 4.0 at some point, which will break
aufs.

This is a follow-up to #9340.