xfs: split the agf_roots and agf_levels arrays

Using arrays of largely unrelated fields that use the btree number
as index is not very robust.  Split the arrays into three separate
fields instead.

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