torture: Stop hanging on panic

By default, the "panic" kernel parameter is zero, which causes the kernel
to loop indefinitely after a panic().  The rcutorture scripting will
eventually kill the corresponding qemu process, but only after waiting
for the full run duration plus a few minutes.  This works, but delays
notifying the developer of the failure.

This commit therefore causes the rcutorture scripting to pass the
"panic=-1" kernel parameter, which caused the kernel to instead
unceremoniously shut down immediately.  This in turn causes qemu to
terminate, so that if all of the runs in a given batch panic(), the
rcutorture scripting can immediately proceed to the next batch.

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