lkdtm: add tests for additional page permissions

Testing execution and access of userspace from the kernel is needed for
validating things like Intel's SMEP and SMAP protections. Additionally,
add an explicit test for validating that RO page permissions have been
set for the RO data area.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>