rcutorture: Check for no up/down readers at task level

rcutorture: Check for no up/down readers at task level

The design of testing of up/down readers such as srcu_down_read()
and srcu_up_read() assumes that these are tested only by the
rcu_torture_updown() kthread, and never by the rcu_torture_reader()
kthread.  Because we all know which road is paved with good intentions,
this commit adds WARN_ON_ONCE() to verify that things are going to plan.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>