Convert write_cache_pages() to an iterator v2

[Christoph Hellwig]

Hi all,

this is basically a report of willy's series of the same name for June.
I picked the lastest version from his (now apparently defunct) git tree,
rebased it to mainline (no coflict, neither for linux-next), reordered
the new fields in struct writeback_control to document what is interface
vs internal, and temporarily dropped the iomap patch due to a conflict
in the VFS tree.

willy: let me know if me reposting it like this is fine, or if you
want me to stop.  I'd just really like to see it merged :)
Note that patch 4 is missing your signoff, so we'd need that before
proceeding anyway.

The original cover letter is below:

Dave Howells doesn't like the indirect function call imposed by
write_cache_pages(), so refactor it into an iterator.  I took the
opportunity to add the ability to iterate a folio_batch without having
an external variable.

This is against next-20230623.  If you try to apply it on top of a tree
which doesn't include the pagevec removal series, IT WILL CRASH because
it won't reinitialise folio_batch->i and the iteration will index out
of bounds.

I have a feeling the 'done' parameter could have a better name, but I
can't think what it might be.

Diffstat:
 include/linux/pagevec.h   |   18 ++
 include/linux/writeback.h |   24 +++
 mm/page-writeback.c       |  313 +++++++++++++++++++++++++---------------------
 3 files changed, 211 insertions(+), 144 deletions(-)