There are many case where the keywords `free` and `available`
are confused in their usage.
For most cases, `free` space is the amount of free space left
on the whole disk, and not just in allocation while
`available` space is the amount of free space left in the
allocated disk space.
What the user/sno wants to see is not the free space but the
available space. To the SNO, free space is the free space
left in the allocated disk space.
Because of this confusion, the multinode dashboard displays
the `free` disk space instead of the free space in the
allocated disk space https://github.com/storj/storj/issues/5248
While the storagenode dashboard shows the correct free space
in the allocation.
This change fixes the wrong free disk space. I also added a
few comments to make a distinction between the `free`
and `available` fields in the `DiskSpace*` structs.
Change-Id: I11b372ca53a5ac05dc3f79834c18f85ebec11855
