refactor the iomap writeback code v5

[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 testing on xfs, and gets as far as mainline for
gfs2 (crashes in generic/361).

Changes since v4:
 - add back includes needed in some configs
 - drop an include not needed after a code move

Changes since v3:
 - add a patch to drop unused includes
 - drop the iomap_writepage_ctx renaming - we should do this separately and
   including the variable names if desired
 - add a comment about special casing of holes in iomap_writeback_range
 - split the cleanups to iomap_read_folio_sync into a separate prep patch
 - explain the IOMAP_HOLE check in xfs_iomap_valid
 - explain the iomap_writeback_folio later folio unlock vs dropbehind
 - some cargo culting for the #$W# RST formatting
 - "improve" the documentation coverage a bit

Changes since v2:
 - rename iomap_writepage_ctx to iomap_writeback_ctx
 - keep local map_blocks helpers in XFS
 - allow buildinging the writeback and write code for !CONFIG_BLOCK

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 |   57 +-
 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/Makefile                              |    6 
 fs/iomap/buffered-io.c                         |  553 +++++++------------------
 fs/iomap/direct-io.c                           |    5 
 fs/iomap/fiemap.c                              |    3 
 fs/iomap/internal.h                            |    1 
 fs/iomap/ioend.c                               |  220 +++++++++
 fs/iomap/iter.c                                |    1 
 fs/iomap/seek.c                                |    4 
 fs/iomap/swapfile.c                            |    3 
 fs/iomap/trace.c                               |    1 
 fs/iomap/trace.h                               |    4 
 fs/xfs/xfs_aops.c                              |  212 +++++----
 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                          |   82 ++-
 25 files changed, 705 insertions(+), 609 deletions(-)