nfs: fix 'dangling' files in lossy sessions

nfs: fix 'dangling' files in lossy sessions

In case of lossy connections the NFS state would properly clean up
transactions, including file transactions. However for files the
state was never set to 'truncated', leading to files to stay 'active'.

This would lead these files staying in the NFS's state. In long running
sessions with lots of files this would lead to performance and memory
use issues.

This patch cleans truncates the file that was being transmitted when
a file transaction is being closed.

Based on 65e9a7c31cc68bdb1fb3e1412b0a56260265c608