lightnvm: cleanup in hins

* Move to explicit naming of structs and enum.
* Remove malloc for user space hint.
* Refactor nvm_send_hint and nvm_bio_hint to have less indentations.
* Removed ability to submit multiple hints in one user space ioctl. This
  is be put back in when we get a need. This isn't strictly necessary for
  latency hints.
* Rename some variables, of which their parent already gave them
  meaning.