Send proper block to old clients for swap_node calls

The legacy code added in commit

d879a539cd - "Add minetest.swap_node"

for sending the whole mapblock to older clients on the case of a node
modification with swap_node, had the problem that the block chosen to be
sent to the client was referenced with node coordinates and not with
block coordinates, resulting in getting the wrong block sent to the client.

src/server.cpp

@@ -2102,7 +2102,7 @@ void Server::sendAddNode(v3s16 p, MapNode n, u16 ignore_id,
 				if (client->net_proto_version <= 21) {
 					// Old clients always clear metadata; fix it
 					// by sending the full block again.
-					client->SetBlockNotSent(p);
+					client->SetBlockNotSent(getNodeBlockPos(p));
 				}
 			}
 		}