rcutorture: Add parameters to control polled/conditional wait interval

rcutorture: Add parameters to control polled/conditional wait interval

This commit adds rcutorture module parameters gp_cond_wi, gp_cond_wi_exp,
gp_poll_wi, and gp_poll_wi_exp to control the wait interval for
conditional, conditional expedited, polled, and polled expedited grace
periods, respectively.  When rcu_torture_writer() is testing these types
of grace periods, hrtimers are used to randomly wait up to the specified
number of microseconds, but with nanosecond granularity.

In the case of conditional grace periods (get_state_synchronize_rcu()
and cond_synchronize_rcu(), for example) there is just one
wait.  For polled grace periods (start_poll_synchronize_rcu() and
poll_state_synchronize_rcu(), for example), there is a repeated series
of waits until the grace period ends.

For normal grace periods, the default is 16 jiffies (for example, 16,000
microseconds on a HZ=1000 system) and for expedited grace periods the
default is 128 microseconds.

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