Removes all traces of mds_req, mds_rep, ost_req, and ost_rep. All subsystems

now use the functions in rpc/pack_generic.c, which are based around
struct lustre_msg.  That file is small and easily understood, see it for more
details.

Now that everyone uses the same functions, the code can probably be refactored
more sensibly.  This is left as an exercise for the reader.

Other, cleanups:
- we don't need to memset() after OBD_ALLOC
- the MDS and OST request and reply packets were so similar that they've been
merged, into struct mds_body and ost_body respectively.