xfs: refactor xfs_mount_zones

xfs: refactor xfs_mount_zones

xfs_mount_zones has grown a bit too big and unorganized.  Split the
zone reporting loop into a separate helper, hiding the rtg variable
there.  Print the mount message last, and also keep the VFS writeback
chunk size last instead of in the middle of the logic to calculate
the free/available blocks.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hans Holmberg <hans.holmberg@wdc.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>