jffs2: Simplify the allocation of slab caches

Use the new KMEM_CACHE() macro instead of direct kmem_cache_create
to simplify the creation of SLAB caches.
And change cache name from 'jffs2_tmp_dnode' to 'jffs2_tmp_dnode_info'.

Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>