xfs: stricter btree height checking when scanning for btree roots

When we're scanning for btree roots to rebuild the AG headers, make sure
that the proposed tree does not exceed the maximum height for that btree
type (and not just XFS_BTREE_MAXLEVELS).

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Chandan Babu R <chandan.babu@oracle.com>