lkdtm: Provide timing tests for atomic_t vs refcount_t

While not a crash test, this does provide two tight atomic_t and
refcount_t loops for performance comparisons:

	cd /sys/kernel/debug/provoke-crash
	perf stat -B -- cat <(echo ATOMIC_TIMING) > DIRECT
	perf stat -B -- cat <(echo REFCOUNT_TIMING) > DIRECT

Looking a CPU cycles is the best way to example the fast-path (rather
than instruction counts, since conditional jumps will be executed but
will be negligible due to branch-prediction).

Signed-off-by: Kees Cook <keescook@chromium.org>