pnfs-obj: pnfs_osd XDR definitions

* Add the pnfs_osd_xdr.h header

* defintions the pnfs_osd_layout structure including all it's
  sub-types and constants.
* Declare the pnfs_osd_xdr_decode_layout API + all needed
  inline helpers.

* Define the pnfs_osd_deviceaddr structure and all its subtypes and
  constants.
* Declare API for decoding of a pnfs_osd_deviceaddr from XDR stream.

* Define the pnfs_osd_ioerr structure, its substructures and constants.
* Declare API for encoding of a pnfs_osd_ioerr into XDR stream.

* Define the pnfs_osd_layoutupdate structure and its substructures.
* Declare API for encoding of a pnfs_osd_layoutupdate into XDR stream.

[Remove server definitions]
Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>