uorb: reduce RAM usage by avoiding string copies

The lifetime of the string is guaranteed because we never delete
DeviceNode objects and the strings in question are already on the heap.

This frees roughly 2.2KB of RAM on Pixracer

src/modules/uORB/ORBMap.hpp

@@ -62,13 +62,19 @@ public:
 			unlinkNext(_top);
 
 			if (_top->next == nullptr) {
-				free((void *)_top->node_name);
 				free(_top);
 				_top = nullptr;
 				_end = nullptr;
 			}
 		}
 	}
+
+	/**
+	 * Insert an element with a unique name
+	 * @param node_name name of the node. This will not be copied, so the caller has to ensure
+	 *                  the pointer is valid until the node is removed from ORBMap
+	 * @param node
+	 */
 	void insert(const char *node_name, uORB::DeviceNode *node)
 	{
 		Node **p;
@@ -90,7 +96,7 @@ public:
 		}
 
 		_end->next = nullptr;
-		_end->node_name = strdup(node_name);
+		_end->node_name = node_name;
 		_end->node = node;
 	}
 
@@ -124,21 +130,6 @@ public:
 		return nullptr;
 	}
 
-	void unlinkNext(Node *a)
-	{
-		Node *b = a->next;
-
-		if (b != nullptr) {
-			if (_end == b) {
-				_end = a;
-			}
-
-			a->next = b->next;
-			free((void *)b->node_name);
-			free(b);
-		}
-	}
-
 	Node *top() const
 	{
 		return _top;
@@ -150,6 +141,20 @@ public:
 	}
 
 private:
+	void unlinkNext(Node *a)
+	{
+		Node *b = a->next;
+
+		if (b != nullptr) {
+			if (_end == b) {
+				_end = a;
+			}
+
+			a->next = b->next;
+			free(b);
+		}
+	}
+
 	Node *_top;
 	Node *_end;
 };

src/modules/uORB/uORBDevices.cpp

@@ -912,9 +912,9 @@ uORB::DeviceMaster::ioctl(device::file_t *filp, int cmd, unsigned long arg)
 				} else {
 					// add to the node map;.
 #ifdef __PX4_NUTTX
-					_node_map.insert(nodepath, node);
+					_node_map.insert(devpath, node);
 #else
-					_node_map[std::string(nodepath)] = node;
+					_node_map[std::string(devpath)] = node;
 #endif
 				}