Merge branch 'offload-serialize-overlap2' of https://github.com/vincentkfu/fio

* 'offload-serialize-overlap2' of https://github.com/vincentkfu/fio:
  rate-submit: remove code that can never be executed
  docs: enable serialize_overlap with io_submit_mode=offload
  fio: enable overlap checking with offload submission
  init: loosen serialize_overlap restrictions