re-enable IOCB_NOWAIT writes to files v6

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 v5:
 - sample ctime before calling inode_set_ctime_current
 - fix a mild bisection hazard in fat

Changes since v4:
 - replace the S_* flags with an enum indicating either access or
   modification time updates to make the logic less fragile and to
   fix a bug in the previous version

Changes since v3:
 - fix was_dirty_time handling in __mark_inode_dirty for the racy flag
   update case
 - refactor inode_update_timestamps to make the lazytime vs blocking
   logical more clear
 - allow non-blocking timestamp updates for fat

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                      |    8 +-
 fs/fs-writeback.c                     |   33 +++++++---
 fs/gfs2/inode.c                       |    6 +
 fs/inode.c                            |  111 +++++++++++++++++++++-------------
 fs/internal.h                         |    3 
 fs/nfs/inode.c                        |    4 -
 fs/orangefs/inode.c                   |    5 +
 fs/overlayfs/inode.c                  |    2 
 fs/sync.c                             |    4 -
 fs/ubifs/file.c                       |   13 ++-
 fs/xfs/xfs_iops.c                     |   34 +++++++++-
 fs/xfs/xfs_super.c                    |   29 --------
 include/linux/fs.h                    |   27 ++++++--
 include/trace/events/writeback.h      |    6 -
 16 files changed, 182 insertions(+), 111 deletions(-)