tools/sched_ext: scx_userland: fix restart and stats thread lifecycle bugs

tools/sched_ext: scx_userland: fix restart and stats thread lifecycle bugs

Fix three issues in scx_userland's restart path:

- exit_req is not reset on restart, causing sched_main_loop() to exit
  immediately without doing any scheduling work.

- stats_printer thread handle is local to spawn_stats_thread(), making
  it impossible to join from main(). Promote it to file scope.

- The stats thread continues reading skel->bss after the skeleton is
  destroyed on restart, causing a use-after-free. Join the stats thread
  before destroying the skeleton to ensure it has exited.

Signed-off-by: David Carlier <devnexen@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>