Per default, "rawBridge" is shown here, which does not tell the user
much about the filesystem. Set to "loopback" as shown below.

Before:

  $ df -T
  Filesystem                 Type            1K-blocks       Used Available Use% Mounted on
  rawBridge                  fuse.rawBridge  237371192  149873220  75370536  67% /home/jakob/go/src/github.com/hanwen/go-fuse/example/loopback/b

After:

  $ df -T
  Filesystem                 Type           1K-blocks       Used Available Use% Mounted on
  loopback                   fuse.loopback  237371192  149885928  75357828  67% /home/jakob/go/src/github.com/hanwen/go-fuse/example/loopback/b

Change-Id: Iaaa18b4193b31567f6589669e8c443715afbd362