rcutorture: Use rcu_gp_slow_register/unregister() only for rcutype test

rcutorture: Use rcu_gp_slow_register/unregister() only for rcutype test

The rcu_gp_slow_register/unregister() is only useful in tests where
torture_type=rcu, so this commit therefore generates ->gp_slow_register()
and ->gp_slow_unregister() function pointers in the rcu_torture_ops
structure, and slows grace periods only when these function pointers
exist.

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>