rcutorture: Fix rcu_torture_one_read()/rcu_torture_writer() data race

The ->rtort_pipe_count field in the rcu_torture structure checks for
too-short grace periods, and is therefore read by rcutorture's readers
while being updated by rcutorture's writers.  This commit therefore
adds the needed READ_ONCE() and WRITE_ONCE() invocations.

This data race was reported by KCSAN.  Not appropriate for backporting
due to failure being unlikely and due to this being rcutorture.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>