rcuscale: Add minruntime module parameter

rcuscale: Add minruntime module parameter

By default, rcuscale collects only 100 points of data per writer, but
arranging for all kthreads to be actively collecting (if not recording)
data during the time that any kthread might be recording.  This works
well, but does not allow much time to bring external performance tools
to bear.  This commit therefore adds a minruntime module parameter
that specifies a minimum data-collection interval in seconds.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>