fortify: Allow strlen() and strnlen() to pass compile-time known lengths

fortify: Allow strlen() and strnlen() to pass compile-time known lengths

Under CONFIG_FORTIFY_SOURCE, it is possible for the compiler to perform
strlen() and strnlen() at compile-time when the string size is known.
This is required to support compile-time overflow checking in strlcpy().

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