Fix a few things related to Flex File Layout. The XDR needed to be fixed
for the DeviceInfo. The layout was changed to provide non-empty string for
uid/gid. These aren't used by the client, but the Linux client doesn't like
empty strings. Also, get rid of the No_IO_through_MDS flag, since this makes
the Linux client loop internally. It is just a hint, so I believe that is
a Linux Flex File Layout driver bug.

The Linux client still doesn't like the Flex File Layout for some reason,
but the XDR looks ok on the wire and Linux falls back to I/O through the
MDS, so at least the Linux mounts work now.