Convert write_cache_pages() to an iterator v4

[Christoph Hellwig]

Hi all,

this is basically a evolution of the series Matthew Wilcox originally
set in June.  Based on comments from Jan a Brian this now actually
untangles some of the more confusing conditional in the writeback code
before refactoring it into the iterator.  Because of that all the
later patches need a fair amount of rebasing and I've not carried any
reviewed-by over.

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.

Changes since v3:
 - various commit log spelling fixes
 - remove a statement from a commit log that isn't true any more with the
   changes in v3
 - rename a function
 - merge two helpers

Diffstat:
 include/linux/pagevec.h   |   18 ++
 include/linux/writeback.h |   19 ++
 mm/page-writeback.c       |  328 +++++++++++++++++++++++++---------------------
 3 files changed, 215 insertions(+), 150 deletions(-)