lkdtm: Add a test for STACKLEAK

Introduce an lkdtm test for the STACKLEAK feature: check that the
current task stack is properly erased (filled with STACKLEAK_POISON).

Signed-off-by: Alexander Popov <alex.popov@linux.com>
Signed-off-by: Tycho Andersen <tycho@tycho.ws>
Tested-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>