re-enable IOCB_NOWAIT writes to files v3

[Christoph Hellwig]

Hi all,

commit 66fa3cedf16a ("fs: Add async write file modification handling.")
effectively disabled IOCB_NOWAIT writes as timestamp updates currently
always require blocking, and the modern timestamp resolution means we
always update timestamps.  This leads to a lot of context switches from
applications using io_uring to submit file writes, making it often worse
than using the legacy aio code that is not using IOCB_NOWAIT.

This series allows non-blocking updates for lazytime if the file system
supports it, and adds that support for XFS.

Changes since v2:
 - drop patches merged upstream
 - adjust for the inode state accesors
 - keep a check in __writeback_single_inode instead of exercising
   potentially undefined behavior
 - more spelling fixes

Changes since v1:
 - more regular numbering of the S_* flags
 - fix XFS to actually not block
 - don't ignore the generic_update_time return value in
   file_update_time_flags
 - fix the sync_lazytime return value
 - fix an out of data comment in btrfs
 - fix a race that would update i_version before returning -EAGAIN in XFS

Diffstat:
 Documentation/filesystems/locking.rst |    2 
 Documentation/filesystems/vfs.rst     |    6 ++
 fs/btrfs/inode.c                      |   11 +++-
 fs/fat/misc.c                         |    3 +
 fs/fs-writeback.c                     |   52 ++++++++++++++++---
 fs/gfs2/inode.c                       |    6 +-
 fs/inode.c                            |   89 +++++++++++++++++++---------------
 fs/internal.h                         |    3 -
 fs/nfs/inode.c                        |    4 -
 fs/orangefs/inode.c                   |    8 ++-
 fs/overlayfs/inode.c                  |    3 +
 fs/sync.c                             |    4 -
 fs/ubifs/file.c                       |   11 ++--
 fs/xfs/xfs_iops.c                     |   35 ++++++++++++-
 fs/xfs/xfs_super.c                    |   29 -----------
 include/linux/fs.h                    |   19 ++++---
 include/trace/events/writeback.h      |    6 --
 17 files changed, 182 insertions(+), 109 deletions(-)