sched_ext: Minor cleanups to scx_task_iter

sched_ext: Minor cleanups to scx_task_iter

- Use memset() in scx_task_iter_start() instead of zeroing fields individually.

- In scx_task_iter_next(), move __scx_task_iter_maybe_relock() after the batch
  check which is simpler.

- Update comment to reflect that tasks are removed from scx_tasks when dead
  (commit 7900aa699c34 ("sched_ext: Fix cgroup exit ordering by moving
  sched_ext_free() to finish_task_switch()")).

No functional changes.

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