lkdtm: fix memory leak of val

This case is supposed to read from a page after after it is freed, but
it missed freeing val if we are not able to get a free page.

Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Signed-off-by: Kees Cook <keescook@chromium.org>