Have the total file size of a volume into struct ltfs_index

[piste-jp]

Is your feature request related to a problem? Please describe.
I would like to know how the total file size in a LTFS tape (accumulated logical size without tape drive compression and size of indexes).

Describe the solution you'd like
Add a field into struct ltfs_index.

[mrichmon]

How would this be a benefit over using standard filesystem tools such as du or df to calculate the file sizes on the volume?

The concern with having a filesystem-wide value that holds the sum of the file sizes on the volume is that maintaining the accuracy of the total is fairly complex and will likely require both a global lock, and some interpolation of the seek and write operations per open file.