sched_ext: Properly mark SCX-internal migrations via sticky_cpu

sched_ext: Properly mark SCX-internal migrations via sticky_cpu

Reposition the setting and clearing of sticky_cpu to better define the
scope of SCX-internal migrations.

This ensures @sticky_cpu is set for the entire duration of an internal
migration (from dequeue through enqueue), making it a reliable indicator
that an SCX-internal migration is in progress. The dequeue and enqueue
paths can then use @sticky_cpu to identify internal migrations and skip
BPF scheduler notifications accordingly.

This prepares for a later commit fixing the ops.dequeue() semantics.
No functional change intended.

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