NFSD: Record each NFSv4 call's session slot index

Help the client resolve the race between the reply to an
asynchronous COPY reply and the associated CB_OFFLOAD callback by
planting the session, slot, and sequence number of the COPY in the
CB_SEQUENCE contained in the CB_OFFLOAD COMPOUND.

Suggested-by: Trond Myklebust <trondmy@hammerspace.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>