BACKPORT: FROMGIT: erofs: fix up compacted indexes for block size < 4096

Previously, the block size always equaled to PAGE_SIZE, therefore
`lclusterbits` couldn't be less than 12.

Since sub-page compressed blocks are now considered, `lobits` for
a lcluster in each pack cannot always be `lclusterbits` as before.
Otherwise, there is no enough room for the special value
`Z_EROFS_VLE_DI_D0_CBLKCNT`.

To support smaller block sizes, `lobits` for each compacted lcluster is
now calculated as:
   lobits = max(lclusterbits, ilog2(Z_EROFS_VLE_DI_D0_CBLKCNT) + 1)

Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20231206091057.87027-4-hsiangkao@linux.alibaba.com



Bug: 318378021
Change-Id: Iacd89e2b33ddf39ea40b90e88a2bf99bb5a83b31
(cherry picked from commit 8d2517aa
 https: //git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs.git dev)
[dhavale: resolved conflicts in zmap.c due to older naming of constants
and updated commit message also to use the older names]
Signed-off-by: Sandeep Dhavale <dhavale@google.com>