lkdtm: adjust usercopy tests to bypass const checks

The hardened usercopy is now consistently avoiding checks against const
sizes, since we really only want to perform runtime bounds checking
on lengths that weren't known at build time. To test the hardened usercopy
code, we must force the length arguments to be seen as non-const.

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