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 --git a/arch/x86/Kconfig.debug b/arch/x86/Kconfig.debug
index 2e1e3af..fa55514 100644
--- a/arch/x86/Kconfig.debug
+++ b/arch/x86/Kconfig.debug
@@ -220,9 +220,9 @@
 	  This option will cause struct boot_params to be exported via debugfs.
 
 config CPA_DEBUG
-	bool "CPA self test code"
+	bool "CPA self-test code"
 	depends on DEBUG_KERNEL
 	help
-	  Do change_page_attr self tests at boot.
+	  Do change_page_attr() self-tests every 30 seconds.
 
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