erofs: switch on-disk header `erofs_fs.h` to MIT license

erofs: switch on-disk header `erofs_fs.h` to MIT license

Switch to the permissive MIT license to make the EROFS on-disk format
more interoperable across various use cases.

It was previously recommended by the Composefs folks, for example:
https://github.com/composefs/composefs/pull/216#discussion_r1356409501

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Acked-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Acked-by: Yue Hu <zbestahu@gmail.com>
Acked-by: Chao Yu <chao@kernel.org>
Acked-by: Jianan Huang <jnhuang95@gmail.com>
Acked-by: Chunhai Guo <guochunhai@vivo.com>
Acked-by: Hongbo Li <lihongbo22@huawei.com>