xfs_repair: simplify free space btree calculations in init_freespace_cursors

xfs_repair: simplify free space btree calculations in init_freespace_cursors

Add a summary variable to the bulkload structure so that we can track
the number of blocks that have been reserved for a particular (btree)
bulkload operation.  Doing so enables us to simplify the logic in
init_freespace_cursors that deals with figuring out how many more blocks
we need to fill the bnobt/cntbt properly.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>