Btrfs: wire up the free space tree to the extent tree

Btrfs: wire up the free space tree to the extent tree

The free space tree is updated in tandem with the extent tree. There are
only a handful of places where we need to hook in:

1. Block group creation
2. Block group deletion
3. Delayed refs (extent creation and deletion)
4. Block group caching

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>
Orabug: 26274676

(cherry picked from commit 1e144fb8f4a4d6d6d88c58f87e4366e3cd02ab72)
Signed-off-by: Shan Hai <shan.hai@oracle.com>
Reviewed-by: Somasundaram Krishnasamy <somasundaram.krishnasamy@oracle.com>