xfs: separate space btree structures in xfs_ondisk.h

Create a separate section for space management btrees so that they're
not mixed in with file structures.  Ignore the dsb stuff sprinkled
around for now, because we'll deal with that in a subsequent patch.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>