dma-buf: make kobj_type structure constant

Since commit ee6d3dd4 ("driver core: make kobj_type constant.")
the driver core allows the usage of const struct kobj_type.

Take advantage of this to constify the structure definition to prevent
modification at runtime.
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230217-kobj_type-dma-buf-v1-1-b84a3616522c@weissschuh.net

drivers/dma-buf/dma-buf-sysfs-stats.c

@@ -112,7 +112,7 @@ static void dma_buf_sysfs_release(struct kobject *kobj)
 	kfree(sysfs_entry);
 }
 
-static struct kobj_type dma_buf_ktype = {
+static const struct kobj_type dma_buf_ktype = {
 	.sysfs_ops = &dma_buf_stats_sysfs_ops,
 	.release = dma_buf_sysfs_release,
 	.default_groups = dma_buf_stats_default_groups,