drivers/misc/lkdtm: add new file in LKDTM to test fortified strscpy

drivers/misc/lkdtm: add new file in LKDTM to test fortified strscpy

This new test ensures that fortified strscpy has the same behavior than
vanilla strscpy (e.g.  returning -E2BIG when src content is truncated).
Finally, it generates a crash at runtime because there is a write overflow
in destination string.

Link: https://lkml.kernel.org/r/20201122162451.27551-5-laniel_francis@privacyrequired.com
Signed-off-by: Francis Laniel <laniel_francis@privacyrequired.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Daniel Axtens <dja@axtens.net>
Cc: Daniel Micay <danielmicay@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>