xfs: refactor free space btree record initialization

Refactor the code that populates the free space btrees of a new AG so
that we can avoid code duplication once things start getting
complicated.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>