Also pull nfs4_pnfs_ds_addr and nfs4_pnfs_ds to generic pnfs.

They can all be reused by flexfile layout as well.
Reviewed-by: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: Peng Tao <tao.peng@primarydata.com>
Signed-off-by: Tom Haynes <Thomas.Haynes@primarydata.com>