aio fixes for use after free and freeze protection

[Christoph Hellwig]

Hi Linus, hi Al,

below is the new version of the aio fix(es).

Patch one just holds an additional file reference over AIO ops.  This
one is minimally invasive and a clear 4.9 and stable candidate.

The next one drops the never implemented aio_fsync methods because it
makes my life easier later on.  No user visible change as we always
ended up returning EINVAL anyway.

Patch three refators the aio code so that it's not a spaghetti monster,
and patch four is Jan's original free patch rebased on top of this whole
stack.