mm: allow page poisoning to be enabled by default.

Add a config option to enable page poisoning by
default. The kernel command line option "page_poison"
can be used to change the behaviour during boot.

Change-Id: Ie70763841191a722b1c6125dfad119a29ed0f605
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>