LU-928 fid: add comments describing different FIDs

LU-928 fid: add comments describing different FIDs

Add comments to the code describing various FID types, from
http://wiki.lustre.org/index.php/Architecture_-_Interoperability_fids_zfs

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I325b48a0e85fb25ed8c3a3709e623978969d8d4a
Reviewed-on: http://review.whamcloud.com/4102
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Tested-by: Hudson
Reviewed-by: Ned Bass <bass6@llnl.gov>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>