[WIP] erofs/erofs-utils: support multiple block compression

[Gao Xiang]

Hi all,

For anyone who interests, I've made a preliminary kernel implementation
and it seems to work now. I'm cleaning up these patchset and hopefully
send it out later. The code can be fetched from:

git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git -b experimental-bigpcluster

and
git://git.kernel.org/pub/scm/linux/kernel/git/xiang/linux.git -b erofs/bigpcluster

The benefit has been showed in the previous email, see:
https://lkml.kernel.org/r/20201230101239.GA3282742@xiangao.remote.csb

What I currently saw is that the sequential read can have slightly
benefit compared with EROFS 4k case (mainly due to CR improvement),
correspondingly rand9m has some drop, that is understandable.

The next step would be to enable compact indexes for big pclusters.
That will boost up the read performance even further. Also there are
still some finer optimization for big pclusters as well. I will seek
more extra time on optimizing case-by-case. I hope the big pcluster
on-disk format can be finalized in the next month, and upstream the
first runtime version for the next 5.13 cycle.

Finally, first EROFS LZMA compression implementation will be planned
for the 5.14 cycle if no strange happens. With bigpcluster feature
landed, EROFS LZMA compression can be more powerful then since such
end users care more about CR rather than random performance instead.

Thanks,
Gao Xiang