scftorture: Allow zero weight to exclude an smp_call_function*() category

This commit reworks the weighting calculations to allow zero to
be specified to disable a given weight.  For example, specifying the
scftorture.weight_resched=0 kernel boot parameter without specifying a
non-zero value for any of the other scftorture.weight_* parameters would
provide the default weights for the others, but would refrain from doing
any resched-based IPIs.

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