This change adds the --direct-io flag to the mount. This means the
page cache is completely bypassed for reads and writes. No read-ahead
takes place. Shared mmap is disabled.
This is useful to accurately read files which may change length
frequently on the source.
Before this change, the current working directory could disappear
according to the Linux kernel.
This was caused by mount returning different nodes with the same
information in.
This change uses vfs.Node.SetSys to cache the information so we always
return the same node.