xfs: count free space btree blocks when scrubbing pre-lazysbcount fses

Since agf_btreeblks didn't exist before the lazysbcount feature, the fs
summary count scrubber needs to walk the free space btrees to determine
the amount of space being used by those btrees.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Gao Xiang <hsiangkao@redhat.com>