convert write_cache_pages() to an iterator v6

[Christoph Hellwig <hch@lst.de>]

Hi all,

this is an evolution of the series Matthew Wilcox originally sent in June
2023, which has changed quite a bit since and now has a while based
iterator.

Note that in this version two patches are so different from the previous
version that I've not kept any Reviews or Acks for them, even if the
final result look almost the same as the previous patches with the
incremental patch on the list.

Changes since v5:
 - completely reshuffle the series to directly prepare for the
   writeback_iter() style.
 - don't require *error to be initialized on first call
 - improve various comments
 - fix a bisection hazard where write_cache_pages don't return delayed
   error for a few commits
 - fix a whitespace error
 - drop the iomap patch again for now as the iomap map multiple blocks
   series isn't in mainline yet

Changes since v4:
 - added back the (rebased) iomap conversion now that the conflict is in
   mainline
 - add a new patch to change the iterator

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:
 fs/iomap/buffered-io.c    |   10 -
 include/linux/pagevec.h   |   18 ++
 include/linux/writeback.h |   12 +
 mm/page-writeback.c       |  344 ++++++++++++++++++++++++++--------------------
 4 files changed, 231 insertions(+), 153 deletions(-)