improve use_mm / unuse_mm v2

[Christoph Hellwig]

Hi all,

this series improves the use_mm / unuse_mm interface by better
documenting the assumptions, and my taking the set_fs manipulations
spread over the callers into the core API.

Changes since v1:
 - drop a few patches
 - fix a comment typo
 - cover the newly merged use_mm/unuse_mm caller in vfio