Filesystems usually show the underlying device in the first
column of df. Follow suite and show the original dir, making
"df" and "mount" output more useful.

The actual reason for this change, however, is that it facilitates
running xfstests against go-fuse loopback. The xfstests machinery
checks that the expected device is mounted at the right place.

Commit https://github.com/rfjakob/fuse-xfstests/commit/182841fea487419f493d9b547bf703bee9e39322
adds support for go-fuse loopback to xfstests and depends on this
change.

Before:

  $ 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

After:

  $ df -T
  Filesystem                 Type           1K-blocks       Used Available Use% Mounted on
  a                          fuse.loopback  237371192  150332024  74911732  67% /home/jakob/go/src/github.com/hanwen/go-fuse/example/loopback/b

Change-Id: I3d5a2e7ab021af4eed657e66654b636e08292219