sched_ext: Convert deferred_reenq_locals from llist to regular list

sched_ext: Convert deferred_reenq_locals from llist to regular list

The deferred reenqueue local mechanism uses an llist (lockless list) for
collecting schedulers that need their local DSQs re-enqueued. Convert to a
regular list protected by a raw_spinlock.

The llist was used for its lockless properties, but the upcoming changes to
support remote reenqueue require more complex list operations that are
difficult to implement correctly with lockless data structures. A spinlock-
protected regular list provides the necessary flexibility.

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