x86: delay CPA self-test and repeat it

delay the CPA self-test so that any impact (corruption) of
user-space pagetables can be triggered. Repeat the test
every 30 seconds.

this would have prevented the bug fixed by 8cb2a7c1e95e472b5,
at its source.

Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --cc arch/x86/Kconfig.debug
Simple merge

diff --cc arch/x86/mm/pageattr-test.c
Simple merge