staging: lustre: fld: add fld description documentation

 *

 * lustre/fld/fld_internal.h

 *

 * Subsystem Description:

 * FLD is FID Location Database, which stores where (IE, on which MDT)

 * FIDs are located.

 * The database is basically a record file, each record consists of a FID

 * sequence range, MDT/OST index, and flags. The FLD for the whole FS

 * is only stored on the sequence controller(MDT0) right now, but each target

 * also has its local FLD, which only stores the local sequence.

 *

 * The FLD subsystem usually has two tasks:

 * 1. maintain the database, i.e. when the sequence controller allocates

 * new sequence ranges to some nodes, it will call the FLD API to insert the

 * location information <sequence_range, node_index> in FLDB.

 *

 * 2. Handle requests from other nodes, i.e. if client needs to know where

 * the FID is located, if it can not find the information in the local cache,

 * it will send a FLD lookup RPC to the FLD service, and the FLD service will

 * look up the FLDB entry and return the location information to client.

 *

 *

 * Author: Yury Umanets <umka@clusterfs.com>

 * Author: Tom WangDi <wangdi@clusterfs.com>

 */