torture: Inflict default jitter

This commit enables jitter by default.  It may be manually disabled
by passing "--jitter 0" to kvm.sh.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

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

@@ -48,7 +48,7 @@ resdir=""
 configs=""
 cpus=0
 ds=`date +%Y.%m.%d-%H:%M:%S`
-jitter=0
+jitter="-1"
 
 . functions.sh