sched_ext: Abort directly from the hardlockup handler

In the Linux kernel, the following vulnerability has been resolved:

sched_ext: Abort directly from the hardlockup handler

scx_hardlockup() defers the abort to an irq_work because exit claiming used
to take scx_sched_lock and couldn't run from NMI. The deferral is now
unnecessary - claiming is NMI-safe and asserting ->aborting is exactly what
breaks the live-locks that hard-lock CPUs. Call handle_lockup() directly and
drop the irq_work. This also makes the self-detected case recoverable: the
perf watchdog fires on the hard-locked CPU itself, where a queued irq_work
never runs with IRQs off.

Also fix the return value: %true used to be returned whenever sched_ext was
loaded, suppressing the kernel's hardlockup report even when the abort was
refused. Return %true only when this call initiated the abort.