writeback: allow the file system to override MIN_WRITEBACK_PAGES

writeback: allow the file system to override MIN_WRITEBACK_PAGES

The relatively low minimal writeback size of 4MiB 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.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20251017034611.651385-3-hch@lst.de
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>