rcutorture: Add preempt_count() to rcutorture_one_extend_check() diagnostics

This commit adds the value of preempt_count() to the diagnostics produced
by rcutorture_one_extend_check() to improve debugging.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>