sched/fair: Unify cfs_rq throttling via account_cfs_rq_runtime()

assign_cfs_rq_runtime() during update_curr() sets the resched indicator
and relies on check_cfs_rq_runtime() during pick_next_task() /
put_prev_entity() to throttle the hierarchy once current task is
preempted / blocks.

Per-task throttle, on the other hand, uses throttle_cfs_rq() to simply
propagate the throttle signals, and then relies on task work to
individually throttle the runnable tasks on their way out to the
userspace.

Remove check_cfs_rq_runtime() and unify throttling into
account_cfs_rq_runtime() which only sets the cfs_rq->throttled,
cfs_rq->throttle_count indicators via throttle_cfs_rq() and optionally
adds the task work to the current task (donor) it is on the throttled
hierarchy.

throttle_cfs_rq() requests for sched_cfs_bandwidth_slice() worth of
bandwidth for the current hierarchy that enable it to continue running
uninterrupted when selected. For the rest, it requests a bare minimum of
"1" to ensure some bandwidth is available and pass the
"runtime_remaining > 0" checks once selected.

For SCHED_PROXY_EXEC, a mutex holder cannot exit to userspace without
dropping it first and the mutex_unlock() ensures proxy is stopped before
the mutex handoff which preserves the current semantics for running a
throttled task until it exits to the userspace even if it acts as a
donor.

  [ prateek: rebased on tip, comments, commit message. ]

Reviewed-By: Benjamin Segall <bsegall@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Aaron Lu <ziqianlu@bytedance.com>
Link: https://patch.msgid.link/20260602071005.11942-1-kprateek.nayak@amd.com

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