staging: erofs: add compacted ondisk compression indexes

This patch introduces new compacted compression indexes.

In contract to legacy compression indexes that
   each 4k logical cluster has an 8-byte index,
compacted ondisk compression indexes will have
   amortized 2 bytes for each 4k logical cluster (compacted 2B)
   amortized 4 bytes for each 4k logical cluster (compacted 4B)

In detail, several continuous clusters will be encoded in
a compacted pack with cluster types, offsets, and one blkaddr
at the end of the pack to leave 4-byte margin for better
decoding performance, as illustrated below:
   _____________________________________________
  |___@_____ encoded bits __________|_ blkaddr _|
  0       .                                     amortized * vcnt
  .          .
  .             .                   amortized * vcnt - 4
  .                .
  .___________________.
  |_type_|_clusterofs_|

Note that compacted 2 / 4B should be aligned with 32 / 8 bytes
in order to avoid each pack crossing page boundary.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>