sched_ext: Disable preemption between scx_claim_exit() and kicking helper work

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

sched_ext: Disable preemption between scx_claim_exit() and kicking helper work

scx_claim_exit() atomically sets exit_kind, which prevents scx_error() from

triggering further error handling. After claiming exit, the caller must kick

the helper kthread work which initiates bypass mode and teardown.

If the calling task gets preempted between claiming exit and kicking the

helper work, and the BPF scheduler fails to schedule it back (since error

handling is now disabled), the helper work is never queued, bypass mode

never activates, tasks stop being dispatched, and the system wedges.

Disable preemption across scx_claim_exit() and the subsequent work kicking

in all callers - scx_disable() and scx_vexit(). Add

lockdep_assert_preemption_disabled() to scx_claim_exit() to enforce the

requirement.