drm/i915/selftests: Lock the drm_mm as we search

Hold onto the vm->mutex (to lock the drm_mm) to ensure that the rbtree
is stable as we search it for our scratch node.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200128183458.3860022-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gem/selftests/i915_gem_context.c

@@ -1465,9 +1465,12 @@ static int igt_ctx_readonly(void *arg)
 
 static int check_scratch(struct i915_address_space *vm, u64 offset)
 {
-	struct drm_mm_node *node =
-		__drm_mm_interval_first(&vm->mm,
+	struct drm_mm_node *node;
+
+	mutex_lock(&vm->mutex);
+	node = __drm_mm_interval_first(&vm->mm,
 				       offset, offset + sizeof(u32) - 1);
+	mutex_unlock(&vm->mutex);
 	if (!node || node->start > offset)
 		return 0;