rcutorture: Call preempt_schedule() through static call/key

The rcutorture test suite sometimess triggers a random scheduler
preemption call while simulating a read delay.  Unfortunately, its
direct call to preempt_schedule() bypasses the static call/key filter
used by CONFIG_PREEMPT_DYNAMIC.  This breaks the no-preempt assumption
when the dynamic preemption mode is "none".

For example, rcu_blocking_is_gp() is fooled and abbreviates grace periods
when the CPU runs in no-preempt UP mode.

Fix this by making torture_preempt_schedule() call __preempt_schedule(),
which uses the static call/key.

Reported-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>