sched_ext: Fix starvation of scx_enable() under fair-class saturation

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

sched_ext: Fix starvation of scx_enable() under fair-class saturation

During scx_enable(), the READY -> ENABLED task switching loop changes the

calling thread's sched_class from fair to ext. Since fair has higher

priority than ext, saturating fair-class workloads can indefinitely starve

the enable thread, hanging the system. This was introduced when the enable

path switched from preempt_disable() to scx_bypass() which doesn't protect

against fair-class starvation. Note that the original preempt_disable()

protection wasn't complete either - in partial switch modes, the calling

thread could still be starved after preempt_enable() as it may have been

switched to ext class.

Fix it by offloading the enable body to a dedicated system-wide RT

(SCHED_FIFO) kthread which cannot be starved by either fair or ext class

tasks. scx_enable() lazily creates the kthread on first use and passes the

ops pointer through a struct scx_enable_cmd containing the kthread_work,

then synchronously waits for completion.

The workfn runs on a different kthread from sch->helper (which runs

disable_work), so it can safely flush disable_work on the error path

without deadlock.