torture: Reverse jittering and duration parameters for jitter.sh

Remote rcutorture testing requires that jitter.sh continue to be
invoked from the generated script for local runs, but that it instead
be invoked on the remote system for distributed runs.  This argues
for common jitterstart and jitterstop scripts.  But it would be good
for jitterstart and jitterstop to control the name and location of the
"jittering" file, while continuing to have the duration controlled by
the caller of these new scripts.

This commit therefore reverses the order of the jittering and duration
parameters for jitter.sh, so that the jittering parameter precedes the
duration parameter.

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