fortify: Short-circuit known-safe calls to strscpy()

Replacing compile-time safe calls of strcpy()-related functions with
strscpy() was always calling the full strscpy() logic when a builtin
would be better. For example:

char buf[16];
strcpy(buf, "yes");

would reduce to __builtin_memcpy(buf, "yes", 4), but not if it was:

strscpy(buf, yes, sizeof(buf));

Fix this by checking if all sizes are known at compile-time.

Cc: linux-hardening@vger.kernel.org
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>

string: Convert strscpy() self-test to KUnit

Convert the strscpy() self-test to a KUnit test.

Cc: David Gow <davidgow@google.com>
Cc: Tobin C. Harding <tobin@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/lkml/Y072ZMk/hNkfwqMv@dev-arch.thelio-3990X
Signed-off-by: Kees Cook <keescook@chromium.org>