refactor the iomap writeback code v2

[Christoph Hellwig <hch@lst.de>]

Hi all,

this is an alternative approach to the writeback part of the
"fuse: use iomap for buffered writes + writeback" series from Joanne.
It doesn't try to make the code build without CONFIG_BLOCK yet.

The big difference compared to Joanne's version is that I hope the
split between the generic and ioend/bio based writeback code is a bit
cleaner here.  We have two methods that define the split between the
generic writeback code, and the implemementation of it, and all knowledge
of ioends and bios now sits below that layer.

This version passes basic testing on xfs, and gets as far as mainline
for gfs2 (crashes in generic/361).

Changes since v1:
 - fix iomap reuse in block/zonefs/gfs2 
 - catch too large return value from ->writeback_range
 - mention the correct file name in a commit log
 - add patches for folio laundering
 - add patches for read/modify write in the generic write helpers

Diffstat:
 Documentation/filesystems/iomap/design.rst     |    3 
 Documentation/filesystems/iomap/operations.rst |   51 --
 block/fops.c                                   |   37 +-
 fs/gfs2/aops.c                                 |    8 
 fs/gfs2/bmap.c                                 |   48 +-
 fs/gfs2/bmap.h                                 |    1 
 fs/gfs2/file.c                                 |    3 
 fs/iomap/buffered-io.c                         |  438 ++++++-------------------
 fs/iomap/internal.h                            |    1 
 fs/iomap/ioend.c                               |  220 ++++++++++++
 fs/iomap/trace.h                               |    2 
 fs/xfs/xfs_aops.c                              |  238 +++++++------
 fs/xfs/xfs_file.c                              |    6 
 fs/xfs/xfs_iomap.c                             |   12 
 fs/xfs/xfs_iomap.h                             |    1 
 fs/xfs/xfs_reflink.c                           |    3 
 fs/zonefs/file.c                               |   40 +-
 include/linux/iomap.h                          |   81 ++--
 18 files changed, 630 insertions(+), 563 deletions(-)