sched_ext: TASK_DEAD tasks must be switched out of SCX on ops_disable

scx_ops_disable_workfn() only switches !TASK_DEAD tasks out of SCX while
calling scx_ops_exit_task() on all tasks including dead ones. This can leave
a dead task on SCX but with SCX_TASK_NONE state, which is inconsistent.

If another task was in the process of changing the TASK_DEAD task's
scheduling class and grabs the rq lock after scx_ops_disable_workfn() is
done with the task, the task ends up calling scx_ops_disable_task() on the
dead task which is in an inconsistent state triggering a warning:

  WARNING: CPU: 6 PID: 3316 at kernel/sched/ext.c:3411 scx_ops_disable_task+0x12c/0x160
  ...
  RIP: 0010:scx_ops_disable_task+0x12c/0x160
  ...
  Call Trace:
   <TASK>
   check_class_changed+0x2c/0x70
   __sched_setscheduler+0x8a0/0xa50
   do_sched_setscheduler+0x104/0x1c0
   __x64_sys_sched_setscheduler+0x18/0x30
   do_syscall_64+0x7b/0x140
   entry_SYSCALL_64_after_hwframe+0x76/0x7e
  RIP: 0033:0x7f140d70ea5b

There is no reason to leave dead tasks on SCX when unloading the BPF
scheduler. Fix by making scx_ops_disable_workfn() eject all tasks including
the dead ones from SCX.

Signed-off-by: Tejun Heo <tj@kernel.org>