[REGRESSION] sched/rt: NO_RT_PUSH_IPI causes multi-second PI-boost starvation in pro-audio workloads (dd29c017aed6)

Commit dd29c017aed6 ("sched/rt: Have RT_PUSH_IPI be default off for non
PREEMPT_RT") causes reproducible multi-second audio dropouts on an ordinary
non-PREEMPT_RT desktop running a DAW.

Symptom
-------
Audio cuts to silence for 0.2-3.5s at irregular intervals (roughly one per
2-3 minutes of use) while the machine is otherwise idle and responsive.

Mechanism (as measured, not inferred)
-------------------------------------
REAPER runs one SCHED_RR prio 20 audio thread plus ~15 SCHED_OTHER "livefx"
worker threads. During a stall:

  - the RT audio thread is blocked in rt_mutex_schedule() on a PI mutex
  - exactly ONE livefx worker holds that mutex, has been PI-boosted to RT,
    and is enqueued but never scheduled
  - its schedstat shows run_ns ~0 with wait_ns tracking the entire stall:

        reaper/livefx tid=43910  run=0.0ms  wait=3406.6ms  slices=2
        (during a 3403ms dropout)

  - meanwhile ~17 sibling threads run normally with wait_ns = 0, and no
    thread does meaningful work (largest run was 7.2ms across 3.4s)
  - the machine stays responsive throughout; unrelated SCHED_OTHER work
    proceeds at 84-94% of normal rate with 0-1ms runqueue wait
  - all 16 cores keep taking timer interrupts at normal rates

So a single PI-boosted RT task sits on a runqueue, unmigrated, while other
cores idle. With push disabled nothing hands it to an idle CPU.

Evidence
--------------------------------
  kernel  RT_PUSH_IPI  playing   dropouts >=25ms          xruns
  7.1.8   off (default)  19 min   8  (191ms - 3471ms)   bursts
23/265/434 per min
  7.1.3   on             39 min   0                     0
  7.1.8   on             42 min   0                     0

The third row differs from the first ONLY in the sched feature: Identical
kernel, toolchain, GPU driver, project and audio configuration. At the
observed broken rate ~13 dropouts were expected in that window; zero were
seen, at both 25ms and 10ms detection thresholds (10ms is ~2 missed audio
cycles at quantum 256 / 48kHz).

Workaround
----------
  echo RT_PUSH_IPI > /sys/kernel/debug/sched/features

Applied at boot via a systemd unit.

System
------
  AMD Ryzen 7 5800X (8c/16t), Arch Linux
  kernel 7.1.8-arch1-3 (and 7.1.5, 7.1.6 - all affected; 7.1.3 is clean)
  PipeWire 1.6.8 + pipewire-jack, Focusrite Scarlett 18i20, quantum 256 @ 48kHz
  CONFIG_PREEMPT_RT not set