Report an error when write perm happens while writing an index on DP at sync

[piste-jp]

Summary of changes

This pull request includes following changes or fixes.

• Fix of issue #315

Description

At this time, there is a small chance that LTFS returns good even if all accepted file contents aren't written down to tape when ltfs_write_index() returns WRITE_PERM. So I changed to the code to return error at this condition.

Fixes #315

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• [X] My code follows the style guidelines of this project
• [X] I have performed a self-review of my own code
• [X] I have commented my code, particularly in hard-to-understand areas
• [X] I have made corresponding changes to the documentation
• [X] My changes generate no new warnings
• [X] I have confirmed my fix is effective or that my feature works

[piste-jp]

Merged to master by cherry-pick