UPSTREAM: erofs: reserve physical_clusterbits[]

Formal big pcluster design is actually more powerful / flexable than
the previous thought whose pclustersize was fixed as power-of-2 blocks,
which was obviously inefficient and space-wasting. Instead, pclustersize
can now be set independently for each pcluster, so various pcluster
sizes can also be used together in one file if mkfs wants (for example,
according to data type and/or compression ratio).

Let's get rid of previous physical_clusterbits[] setting (also notice
that corresponding on-disk fields are still 0 for now). Therefore,
head1/2 can be used for at most 2 different algorithms in one file and
again pclustersize is now independent of these.

Link: https://lore.kernel.org/r/20210407043927.10623-2-xiang@kernel.org


Acked-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>

Bug: 201372112
Change-Id: I7db236f06b202949b882a366b347f0c4e9dc0c3e
(cherry picked from commit 54e0b6c873dcbd02b9b479c893f6fba8fcbc6a9c)
Signed-off-by: Huang Jianan <huangjianan@oppo.com>