Enable the slab allocator poisoning

… i.e. slub_debug=P or slub_debug=PF.

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/

It’s not 100% clear if this is a good idea yet, see discussion on #11886 (closed) for details.

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

Related issues

• Related to #11886 (closed)
• Blocked by #12089 (closed)

Original created by @intrigeri on 12090 (Redmine)