sched/core: Dequeue waking proxy donors before reset

proxy_needs_return() resets an active donor while holding blocked_lock.
proxy_reset_donor() invokes scheduling-class callbacks, adding an
unnecessary raw-spinlock nesting.

For an EXT donor, resetting rq->donor first calls put_prev_task_scx()
while SCX_TASK_QUEUED and is_blocked are still set. That path reenqueues
the retained donor through scx_do_enqueue_task(), only for the following
block_task() to dequeue it again.

Split block_task() so the waking donor is first dequeued from its
scheduling class while it is still rq->donor. This lets
dequeue_task_scx() end the donor's running session and clear
SCX_TASK_QUEUED before proxy_reset_donor() invokes put_prev_task_scx().
Keep the generic on_rq state set until donor references are replaced,
then complete the runqueue removal.

This follows the normal sleep ordering, avoids transiently re-enqueuing
the waking donor, and moves the scheduling-class callbacks outside
blocked_lock.

This is a preparatory change to support proxy execution with sched_ext.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260922165445.943315-3-arighi@nvidia.com

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