rcutorture: Use the gp_kthread_dbg operation specified by cur_ops

Despite there being a cur_ops->gp_kthread_dbg(), rcu_torture_writer()
unconditionally invokes vanilla RCU's show_rcu_gp_kthreads().  This is not
at all helpful when some other flavor of RCU is being tested.  This commit
therefore makes rcu_torture_writer() invoke cur_ops->gp_kthread_dbg()
for RCU implementations providing this function.

Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>