kmemcheck: add opcode self-testing at boot

We've had some troubles in the past with weird instructions. This
patch adds a self-test framework which can be used to verify that
a certain set of opcodes are decoded correctly. Of course, the
opcodes which are not tested can still give the wrong results.

In short, this is just a safeguard to catch unintentional changes
in the opcode decoder. It does not mean that errors can't still
occur!

[rebased for mainline inclusion]
Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>

arch/x86/mm/kmemcheck/selftest.h

@@ -0,0 +1,6 @@
+#ifndef ARCH_X86_MM_KMEMCHECK_SELFTEST_H
+#define ARCH_X86_MM_KMEMCHECK_SELFTEST_H
+
+bool kmemcheck_selftest(void);
+
+#endif