Implement EROFS_INODE_FLAT_COMPRESSION

[rapperskull]

Today I tried to extract a EROFS image, but I got stuck since EROFS_INODE_FLAT_COMPRESSION is not implemented. I know you're probably already aware of it, I just wanted to open this issue as a reminder and ask if there's an ETA.

[jakoblell]

Currently we don't have a timeline for it, back in the days when this code was written, EROFS_INODE_FLAT_COMPRESSION_LEGACY was more relevant than EROFS_INODE_FLAT_COMPRESSION. A PR implementing this would definitely be welcome.