sched_ext: When calling ops.dispatch() @prev must be on the same scx_sched

sched_ext: When calling ops.dispatch() @prev must be on the same scx_sched

The @prev parameter passed into ops.dispatch() is expected to be on the
same sched. Passing in @prev which isn't on the sched can spuriously
trigger failures that can kill the scheduler. Pass in @prev iff it's on
the same sched.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>