allow file systems to increase the minimum writeback chunk size v2

[Christoph Hellwig <hch@lst.de>]

Hi all,

The relatively low minimal writeback size of 4MiB leads means that
written back inodes on rotational media are switched a lot.  Besides
introducing additional seeks, this also can lead to extreme file
fragmentation on zoned devices when a lot of files are cached relative
to the available writeback bandwidth.
							         
Add a superblock field that allows the file system to override the
default size, and set it to the zone size for zoned XFS.

Changes since v1:
 - covert the field to a long to match other related writeback code
 - cap the zone XFS writeback size to the maximum extent size
 - write an extensive comment about the tradeoffs of setting the value
 - fix a commit message typo

Diffstat:
 fs/fs-writeback.c         |   26 +++++++++-----------------
 fs/super.c                |    1 +
 fs/xfs/xfs_zone_alloc.c   |   28 ++++++++++++++++++++++++++--
 include/linux/fs.h        |    1 +
 include/linux/writeback.h |    5 +++++
 5 files changed, 42 insertions(+), 19 deletions(-)