276: fix up various issues

[Christoph Hellwig]

On Wed, Jan 04, 2012 at 06:54:46PM +0100, Jan Schmidt wrote:
> This commit fixes bd8ee45c. Changes:
> - added a _require_btrfs helper function
> - check for filefrag with _require_command
> - always use _fail in case of errors
> - added some comments
> - removed $fresh code
> - don't set FSTYP
> 
> Signed-off-by: Jan Schmidt <list.btrfs at jan-o-sch.net>

Thanks, applied.