put the xfs xfile abstraction on a diet v3

[Christoph Hellwig <hch@lst.de>]

Hi all,

this series refactors and simplifies the code in the xfs xfile
abstraction, which is a thing layer on a kernel-use shmem file.

To do this is needs a slighly lower level exports from shmem.c,
which I combined with improving an assert and documentation there.

Now that this series has a fair amount of reviews we also need to
think how we can merge it best.  Should the shmem changes be
merged through the xfs tree?  Should there be a shared branch?

The series is against Linux 6.8-rc2.

Changes since v2:
 - include internal.h in workingset.h to avoid a buildbot warning
 - update the usage comment for shmem_get_folio
 - improve the xfile_load/store documentation
 - don't use highmem for xfiles for real
 - only call filemap_check_wb_err to check for errors on the mapping

Changes since v1:
 - fix reading i_blocks
 - provide wrappers for reading i_size and i_blocks
 - update the xfile load/store documentation
 - improve a commit message
 - use shmem_kernel_file_setup
 - add a missing folio unlock in the hwpoison path in xfile_get_page
 - fix checking for shmem mappings
 - improve I/O error handling (Darrick)
 - convert to folios (partially from Darrick)

Diffstat:
 Documentation/filesystems/xfs/xfs-online-fsck-design.rst |   25 -
 fs/xfs/scrub/rtsummary.c                                 |    6 
 fs/xfs/scrub/trace.h                                     |   81 ++-
 fs/xfs/scrub/xfarray.c                                   |  234 ++++------
 fs/xfs/scrub/xfarray.h                                   |   11 
 fs/xfs/scrub/xfile.c                                     |  345 +++++----------
 fs/xfs/scrub/xfile.h                                     |   62 --
 include/linux/shmem_fs.h                                 |    6 
 include/linux/swap.h                                     |   10 
 mm/filemap.c                                             |    9 
 mm/internal.h                                            |    4 
 mm/shmem.c                                               |   37 +
 mm/workingset.c                                          |    1 
 13 files changed, 349 insertions(+), 482 deletions(-)