vm: region: improve locking rules and semantics; implement region killing

the rules around acquiring locks have been strictly defined and
implemented, and general lock usage has been improved, to fix and
prevent several different issues.

a vm-region is now destroyed in two separate steps:
 1. it is "killed": all mappings are unmapped and deleted, the
    region is removed from its parent, and the region and all of
    its sub-regions are marked as "dead", preventing any
    further actions from being performed with the region.
 2. it is "destroyed": the vm-region object is de-allocated when
    the last reference/handle is closed. the references that this
    region holds to any sub-regions are also released, meaning
    these regions may also be de-allocated too.

syscall/vm-region.c

@@ -63,6 +63,7 @@ kern_status_t sys_vm_region_create(
 
 	object_ref(obj);
 	task_unlock_irqrestore(self, flags);
+	vm_region_lock_irqsave(parent_region, &flags);
 
 	struct vm_region *child = NULL;
 	status = vm_region_create(
@@ -73,6 +74,7 @@ kern_status_t sys_vm_region_create(
 		region_len,
 		prot,
 		&child);
+	vm_region_unlock_irqrestore(parent_region, flags);
 	object_unref(obj);
 
 	if (status != KERN_OK) {
@@ -92,6 +94,39 @@ kern_status_t sys_vm_region_create(
 	return KERN_OK;
 }
 
+kern_status_t sys_vm_region_kill(kern_handle_t region_handle)
+{
+	struct task *self = current_task();
+
+	unsigned long flags;
+	task_lock_irqsave(self, &flags);
+
+	struct object *obj = NULL;
+	handle_flags_t handle_flags = 0;
+	kern_status_t status
+		= task_resolve_handle(self, region_handle, &obj, &handle_flags);
+	if (status != KERN_OK) {
+		task_unlock_irqrestore(self, flags);
+		return status;
+	}
+
+	struct vm_region *region = vm_region_cast(obj);
+	if (!region) {
+		task_unlock_irqrestore(self, flags);
+		return KERN_INVALID_ARGUMENT;
+	}
+
+	object_ref(obj);
+	task_unlock_irqrestore(self, flags);
+
+	vm_region_lock_irqsave(region, &flags);
+	status = vm_region_kill(region, &flags);
+	vm_region_unlock_irqrestore(region, flags);
+	object_unref(obj);
+
+	return status;
+}
+
 kern_status_t sys_vm_region_read(
 	kern_handle_t region_handle,
 	void *dst,