torture: Avoid script syntax error when insufficient CPUs

Parentheses are special to bash, so use an overflow flag that doesn't
use them.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

tools/testing/selftests/rcutorture/bin/kvm.sh

@@ -310,7 +310,7 @@ function dump(first, pastlast)
 			cfr[jn] = cf[j] "." cfrep[cf[j]];
 		}
 		if (cpusr[jn] > ncpus && ncpus != 0)
-			ovf = "(!)";
+			ovf = "-ovf";
 		else
 			ovf = "";
 		print "echo ", cfr[jn], cpusr[jn] ovf ": Starting build. `date`";