xfs: create a standard btree size calculator code

xfs: create a standard btree size calculator code

Source kernel commit: 4ed3f68792f6a9c21a290ae777565e7562a09653

Create a helper to generate AG btree height calculator functions.
This will be used (much) later when we get to the refcount btree.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>