rcutorture: Emulate userspace sojourn during call_rcu() floods

During an actual call_rcu() flood, there would be frequent trips to
userspace (in-kernel call_rcu() floods must be otherwise housebroken).
Userspace execution allows a great many things to interrupt execution,
and rcutorture needs to also allow such interruptions.  This commit
therefore causes call_rcu() floods to occasionally invoke schedule(),
thus preventing spurious rcutorture failures due to other parts of the
kernel becoming irate at the call_rcu() flood events.

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