scftorture: Count reschedule IPIs

Currently, only those IPIs that invoke scftorture's scf_handler()
IPI handler function are counted.  This means that runs exercising
only scftorture.weight_resched will look like they have made no forward
progress, resulting in "GP HANG" complaints from the rcutorture scripting.
This commit therefore increments the scf_invoked_count per-CPU counter
immediately after calling resched_cpu().

Fixes: 1ac78b49 ("scftorture: Add an alternative IPI vector")
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>