To do a credible job of this we need to track the number of free blocks.
We add counters of order allocations free to the indirect blocks so that
we can quickly scan them. We also need a bit of help to count inodes.
Finally I noticed that we were miscalculating the number of slots in the
indirect blocks because we were using the size of the buddy block
header, not the size of the indirect block header.
Signed-off-by: Zach Brown <zab@versity.com>