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 <[e-mail]>
Reviewed-by: Jeff Layton <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

fs/nfsd/nfs4proc.c

···
 			return 0;
 		}
 	}
+	nfsd41_cb_destroy_referring_call_list(cb);
 	return 1;
 }
 
···
 
 	nfsd4_init_cb(&cbo->co_cb, copy->cp_clp, &nfsd4_cb_offload_ops,
 		      NFSPROC4_CLNT_CB_OFFLOAD);
+	nfsd41_cb_referring_call(&cbo->co_cb, &cbo->co_referring_sessionid,
+				 cbo->co_referring_slotid,
+				 cbo->co_referring_seqno);
 	trace_nfsd_cb_offload(copy->cp_clp, &cbo->co_res.cb_stateid,
 			      &cbo->co_fh, copy->cp_count, copy->nfserr);
 	nfsd4_try_run_cb(&cbo->co_cb);
···
 		memcpy(&result->cb_stateid, &copy->cp_stateid.cs_stid,
 			sizeof(result->cb_stateid));
 		dup_copy_fields(copy, async_copy);
+		memcpy(async_copy->cp_cb_offload.co_referring_sessionid.data,
+		       cstate->session->se_sessionid.data,
+		       NFS4_MAX_SESSIONID_LEN);
+		async_copy->cp_cb_offload.co_referring_slotid = cstate->slot->sl_index;
+		async_copy->cp_cb_offload.co_referring_seqno = cstate->slot->sl_seqid;
 		async_copy->copy_task = kthread_create(nfsd4_do_async_copy,
 				async_copy, "%s", "copy thread");
 		if (IS_ERR(async_copy->copy_task))

fs/nfsd/xdr4.h

···
 	__be32			co_nfserr;
 	unsigned int		co_retries;
 	struct knfsd_fh		co_fh;
+
+	struct nfs4_sessionid	co_referring_sessionid;
+	u32			co_referring_slotid;
+	u32			co_referring_seqno;
 };
 
 struct nfsd4_copy {