Truncate of an open file

[brettkettering]

There's a bit of a concern that in an N-1 open through FUSE with O_TRUNC passed and no barrier after open that some procs will truncate data from other procs. This is app misbehavior so I don't know if we should care about this but if we want to protect them, we can always check with nodes have the file open and not truncate their data.

[brettkettering]

Are we going to do this or just expect that the application will only use O_TRUNC for one of the processes that opens a N-1 file?