fix data race in reading Fd and Close of device os.File

specifically running my application with `-race` showed this:

```
==================
WARNING: DATA RACE
Write by goroutine 25:
  os.(*file).close()
      /usr/local/Cellar/go/1.3/libexec/src/pkg/os/file_unix.go:108 +0x19b
  os.(*File).Close()
      /usr/local/Cellar/go/1.3/libexec/src/pkg/os/file_unix.go:97 +0x94
  bazil.org/fuse.(*Conn).Close()
      /go/src/bazil.org/fuse/fuse.go:368 +0x5d
  ...

Previous read by goroutine 37:
  bazil.org/fuse.(*Conn).fd()
      /go/src/bazil.org/fuse/fuse.go:372 +0x80
  bazil.org/fuse.(*Conn).respond()
      /go/src/bazil.org/fuse/fuse.go:842 +0xdf
  bazil.org/fuse.(*DestroyRequest).Respond()
      /go/src/bazil.org/fuse/fuse.go:1431 +0xce
  bazil.org/fuse/fs.(*serveConn).serve()
      /go/src/bazil.org/fuse/fs/serve.go:1168 +0xb8b
```

Since the fd never changes, reading it once on initialization seems sufficient.