rcutorture: Add RCU-bh and RCU-sched support for extended readers

Since there is now a single consolidated RCU flavor, rcutorture
needs to test extending of RCU readers via rcu_read_lock_bh() and
rcu_read_lock_sched().  This commit adds this support, with added checks
(just like for local_bh_enable()) to ensure that rcu_read_unlock_bh()
will not be invoked while interrupts are disabled.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>