rcutorture: Dump trace buffer upon shutdown

When running from the scripts, rcutorture is completely headless,
so there is no way to to manually dump the trace buffer.  This commit
therefore unconditionally dumps the trace buffer upon timed shutdown.
However, if you are using rmmod to end the test, it is still up to you
to manually dump the trace buffer.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>