locktorture: Allow CPU-hotplug to be disabled via --bootargs

The bootparam_hotplug_cpu() bash function was checking for CPU-hotplug
kernel-boot parameters from --bootargs, but that check was specific to
rcutorture ("rcutorture\.onoff_").  This commit therefore makes this
check also work for locktorture ("torture\.onoff_").

Note that rcuperf does not do CPU-hotplug operations, so it is not
necessary to make a similar change for rcuperf.

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