Enable the kernel page allocator poisoning

Following-up on #11886 (closed), extracting this bit so that we can close #11886 (closed) in Tails 2.10.

• http://kernsec.org/wiki/index.php/Kernel_Self_Protection_Project
• https://outflux.net/blog/archives/2016/09/30/security-things-in-linux-v4-6/

This requires CONFIG_PAGE_POISONING=y, requested for the Debian kernel in https://bugs.debian.org/849450.

Feature Branch: bugfix/12354-drop-kexec-memory-wipe

Related issues

• Related to #12354 (closed)
• Blocked by #12122 (closed)
• Blocks #12090 (closed)
• Blocked by #12298 (closed)

Original created by @intrigeri on 12089 (Redmine)