kernel: Make get_cpu_topology() return a pointer to const

The client code is not supposed to change the topology info.
It would be also nice if cpu_topology_node::children was an array of
pointers to const but that would require several const_casts in the
topology tree generation code so it's probably not worth it.

headers/private/kernel/cpu.h

@@ -105,7 +105,7 @@ cpu_ent *get_cpu_struct(void);
 extern inline cpu_ent *get_cpu_struct(void) { return &gCPU[smp_get_current_cpu()]; }
 
 status_t cpu_build_topology_tree(void);
-cpu_topology_node* get_cpu_topology(void);
+const cpu_topology_node* get_cpu_topology(void);
 
 status_t increase_cpu_performance(int delta, bool allowBoost);
 status_t decrease_cpu_performance(int delta);

src/system/kernel/cpu.cpp

@@ -274,7 +274,7 @@ cpu_build_topology_tree(void)
 }
 
 
-cpu_topology_node*
+const cpu_topology_node*
 get_cpu_topology(void)
 {
 	return &sCPUTopology;

src/system/kernel/int.cpp

@@ -391,7 +391,7 @@ restore_interrupts(cpu_status status)
 static
 uint32 assign_cpu(void)
 {
-	cpu_topology_node* node;
+	const cpu_topology_node* node;
 	do {
 		int32 nextID = atomic_add(&sLastCPU, 1);
 		node = get_cpu_topology();

src/system/kernel/scheduler/scheduler.cpp

@@ -751,7 +751,7 @@ scheduler_set_cpu_enabled(int32 cpuID, bool enabled)
 
 
 static void
-traverse_topology_tree(cpu_topology_node* node, int packageID, int coreID)
+traverse_topology_tree(const cpu_topology_node* node, int packageID, int coreID)
 {
 	switch (node->level) {
 		case CPU_TOPOLOGY_SMT:
@@ -805,7 +805,7 @@ build_topology_mappings(int32& cpuCount, int32& coreCount, int32& packageCount)
 		}
 	}
 
-	cpu_topology_node* root = get_cpu_topology();
+	const cpu_topology_node* root = get_cpu_topology();
 	traverse_topology_tree(root, 0, 0);
 
 	cpuToCoreDeleter.Detach();