rcutorture: Warn on bad torture type for built-in tests

When running a built-in rcutorture test, specifying an invalid torture
type results in what looks like a hard hang, with the error messages
hidden by other boot-time output.  This commit therefore executes a
WARN_ON() in this case so that the splat appears just following the
error messages.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>