cleanup the filemap / direct I/O interaction v4

[Christoph Hellwig <hch@lst.de>]

Hi all,

this series cleans up some of the generic write helper calling
conventions and the page cache writeback / invalidation for
direct I/O.  This is a spinoff from the no-bufferhead kernel
project, for which we'll want to an use iomap based buffered
write path in the block layer.

Changes since v3:
 - fix a generic_sync_file that got lost in fuse
 - fix fuse to call fuse_perform_write and not generic_perform_write

Changes since v2:
 - stick to the existing behavior of returning a short write
   if the buffer fallback write or sync fails
 - bring back "fuse: use direct_write_fallback" which accidentally
   got lost in v2

Changes since v1:
 - remove current->backing_dev_info entirely
 - fix the pos/end calculation in direct_write_fallback
 - rename kiocb_invalidate_post_write to
   kiocb_invalidate_post_direct_write
 - typo fixes

diffstat:
 block/fops.c            |   18 ----
 fs/btrfs/file.c         |    6 -
 fs/ceph/file.c          |    6 -
 fs/direct-io.c          |   10 --
 fs/ext4/file.c          |   11 --
 fs/f2fs/file.c          |    3 
 fs/fuse/file.c          |   45 ++---------
 fs/gfs2/file.c          |    6 -
 fs/iomap/buffered-io.c  |    9 +-
 fs/iomap/direct-io.c    |   88 ++++++++-------------
 fs/libfs.c              |   41 ++++++++++
 fs/nfs/file.c           |    6 -
 fs/ntfs/file.c          |    2 
 fs/ntfs3/file.c         |    3 
 fs/xfs/xfs_file.c       |    6 -
 fs/zonefs/file.c        |    4 
 include/linux/fs.h      |    7 -
 include/linux/pagemap.h |    4 
 include/linux/sched.h   |    3 
 mm/filemap.c            |  194 +++++++++++++++++++++---------------------------
 20 files changed, 194 insertions(+), 278 deletions(-)