x86: clean up memory corruption check and add more kernel parameters

The corruption check is enabled in Kconfig by default, but disabled at runtime.

This patch adds several kernel parameters to control the corruption
check's behaviour; these are documented in kernel-parameters.txt.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>