torture: Use numeric taskset argument in jitter.sh

The jitter.sh script has some entertaining awk code to generate a
hex mask from a randomly selected CPU number, which is handed to the
"taskset" command.  Except that this command has a "-c" parameter to
take a comma/dash-separated list of CPU numbers.  This commit therefore
saves a few lines of awk by switching to a single-number CPU list.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>