torture: Avoid format string leak to thead name

Since the torture-test thread creation interface does not include
format string arguments, this commit makes sure the name can never be
accidentally processed as a format string.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>