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

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

schedext: Disable preemption between scxclaim_exit() and kicking helper work

scxclaimexit() atomically sets exitkind, which prevents scxerror() 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 scxclaimexit() and the subsequent work kicking in all callers - scxdisable() and scxvexit(). Add lockdepassertpreemptiondisabled() to scxclaim_exit() to enforce the requirement.