erofs: use kill_anon_super() to kill super in fscache mode

Use kill_anon_super() instead of generic_shutdown_super() since the
mount() in erofs fscache mode uses get_tree_nodev() and associated
anon bdev needs to be freed.

Fixes: 9c0cc9c7

 ("erofs: add 'fsid' mount option")
Suggested-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Jia Zhu <zhujia.zj@bytedance.com>
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220918043456.147-2-zhujia.zj@bytedance.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>