loopback: set filesystem type as shown in "df -T"

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

example/loopback/main.go

@@ -89,6 +89,8 @@ func main() {
 	}
 	opts.Debug = *debug
 	opts.AllowOther = *other
+	// Second column in "df -T" will be shown as "fuse." + Name
+	opts.MountOptions.Name = "loopback"
 	server, err := fs.Mount(flag.Arg(0), loopbackRoot, opts)
 	if err != nil {
 		log.Fatalf("Mount fail: %v\n", err)