Enabled parallel direct IO writes for passthrough examples

Enabled parallel direct IO writes for passthrough examples

All these passthrough examples don't need writes to be serialized.

Actually, most file systems probably handle non serialized parallel
direct writes - the FOPEN_PARALLEL_DIRECT_WRITES flag is just
to avoid a regression for those file system that rely on serialized
DIO writes in fuse kernel. Passthrough file system forward the IO
to another file system, which actually handles that internally -
serialized in fuser kernel is not needed.