scftorture: Implement weighted primitive selection

This commit uses the scftorture.weight* kernel parameters to randomly
chooses between smp_call_function_single(), smp_call_function_many(),
and smp_call_function().  For each variant, it also randomly chooses
whether to invoke it synchronously (wait=1) or asynchronously (wait=0).
The percentage weighting for each option are dumped to the console log
(search for "scf_sel_dump").

This accumulates statistics, which a later commit will dump out at the
end of the run.

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