rcutorture: Print message before invoking ->cb_barrier()

The various ->cb_barrier() functions, for example, rcu_barrier(),
sometimes cause rcutorture hangs.  But currently, the last console
message is the unenlightening "Stopping rcu_torture_stats".  This commit
therefore prints a message of the form "rcu_torture_cleanup: Invoking
rcu_barrier+0x0/0x1e0()" to help point people in the right direction.

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