scftorture: Consolidate scftorture_invoke_one() scf_check initialization

This commit hoists much of the initialization of the scf_check
structure out of the switch statement, thus saving a few lines of code.
The initialization of the ->scfc_in field remains in each leg of the
switch statement in order to more heavily stress memory ordering.

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