ext4: 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.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230209-kobj_type-ext4-v1-1-6865fb05c1f8@weissschuh.net
Signed-off-by: Theodore Ts'o <tytso@mit.edu>