rcuperf: Warn on bad perf type for built-in tests

When running a built-in rcuperf test, specifying an invalid perf 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>