sched_ext: Avoid relocking DSQ during remote consumption

A remote consume unlinks a task from a non-local DSQ before switching
to the task's rq. Leaving p->scx.dsq set makes the following
deactivate_task() reacquire the DSQ lock in scx_dispatch_dequeue() to
clear the transfer state.

Keep p->scx.dsq set while switching rq locks so that a racing dequeue
can find the DSQ and cancel the transfer. After acquiring the source rq
and confirming ownership, clear p->scx.dsq. The subsequent dequeue then
takes its !dsq path without reacquiring the DSQ lock.

Fleet profiles show that roughly 70% of the cycles attributed to
scx_dispatch_dequeue() are spent in queued_spin_lock_slowpath() while
taking dsq->lock.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
Signed-off-by: Tejun Heo <tj@kernel.org>

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