refperf: Change readdelay module parameter to nanoseconds

The current units of microseconds are too coarse, so this commit
changes the units to nanoseconds.  However, ndelay is used only for the
nanoseconds with udelay being used for whole microseconds.  For example,
setting refperf.readdelay=1500 results in an ndelay(500) followed by
a udelay(1).

Suggested-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>