fs/erofs: Add support for the EROFS

fs/erofs: Add support for the EROFS

The EROFS [1] is a lightweight read-only filesystem designed for performance
which has already been shipped in most Linux distributions as well as widely
used in several scenarios, such as Android system partitions, container
images and rootfs for embedded devices.

This patch brings in the EROFS uncompressed support. Now, it's possible to
boot directly through GRUB with an EROFS rootfs.

Support for the EROFS compressed files will be added later.

[1] https://erofs.docs.kernel.org

Signed-off-by: Yifan Zhao <zhaoyifan@sjtu.edu.cn>
Tested-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>