btrfs: make kobj_type structures 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 definitions to prevent
modification at runtime.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>