rcutorture: Add diagnostic for unscheduled system shutdown

Currently, rcutorture can terminate via rmmod, via self-shutdown,
via something else shutting the system down, or of course the usual
catastrophic termination.  The first two get flagged, so this commit adds
a message for the third.  For the fourth, your warranty is void as always.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>