erofs: add anonymous inode caching metadata for data blobs

Introduce one anonymous inode for data blobs so that erofs can cache
metadata directly within such anonymous inode.

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220425122143.56815-14-jefflexu@linux.alibaba.com


Acked-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>