refperf: Add read-side delay module parameter

This commit adds a refperf.readdelay module parameter that controls the
duration of each critical section.  This parameter allows gathering data
showing how the performance differences between the various primitives
vary with critical-section length.

Cc: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>