sched_ext: scx_dsq_move() should validate the task belongs to the right scheduler

sched_ext: scx_dsq_move() should validate the task belongs to the right scheduler

scx_bpf_dsq_move[_vtime]() calls scx_dsq_move() to move task from a DSQ to
another. However, @p doesn't necessarily have to come form the containing
iteration and can thus be a task which belongs to another scx_sched. Verify
that @p is on the same scx_sched as the DSQ being iterated.

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