Tiny correction for sync fail

[piste-jp]

Summary of changes

Set negative value correctly when sync failure happens.

• Additional fix of issue LTFS might lose data when a permanent write error happens in writing index ion DP #315 (Report an error when write perm happens while writing an index on DP at sync #316)

Description

Currently error code of the sync failure is not correct (return a positive value). But it is wrong, need to return negative value.

Type of change

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

Checklist:

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

[piste-jp]

Cherry picked to master