sched_ext: Implement scx_bpf_dsq_reenq() for user DSQs

sched_ext: Implement scx_bpf_dsq_reenq() for user DSQs

scx_bpf_dsq_reenq() currently only supports local DSQs. Extend it to support
user-defined DSQs by adding a deferred re-enqueue mechanism similar to the
local DSQ handling.

Add per-cpu deferred_reenq_user_node/flags to scx_dsq_pcpu and
deferred_reenq_users list to scx_rq. When scx_bpf_dsq_reenq() is called on a
user DSQ, the DSQ's per-cpu node is added to the current rq's deferred list.
process_deferred_reenq_users() then iterates the DSQ using the cursor helpers
and re-enqueues each task.

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