rcutorture: Use DEFINE_STATIC_SRCU()

Use DEFINE_STATIC_SRCU() to simplify the rcutorture.c SRCU test code.

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