xfs: set s_min_writeback_pages for zoned file systems

xfs: set s_min_writeback_pages for zoned file systems

Set s_min_writeback_pages to the zone size, so that writeback always
writes up to a full zone.  This ensures that writeback does not add
spurious file fragmentation when writing back a large number of
files that are larger than the zone size.

Fixes: 4e4d52075577 ("xfs: add the zoned space allocator")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20251017034611.651385-4-hch@lst.de
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>