[pnfs] draft-19 cb_compound

[Benny Halevy]

This patchset updates cb_compound on server and client to draft-19
(actually 15, it hasn't changed since)

The patches have been applied onto the nfs41-d19 branch.
pnfs-d19 and panlayout-d19 were rebased onto it.

[PATCH 1/4] nfsd41: remove unused initialization of callback ident in cb_recall
[PATCH 2/4] nfsd41: update cb_compound encoding to draft-15
[PATCH 3/4] nfs41: update cb_compound decoding to draft-15
[PATCH 4/4] nfs41: get rid of cb_compound_hdr_arg.callback_ident