erofs: add encoded extent on-disk definition

erofs: add encoded extent on-disk definition

Previously, EROFS provided both (non-)compact compressed indexes to
keep necessary hints for each logical block, enabling O(1) random
indexing.  This approach was originally designed for small compression
units (e.g., 4KiB), where compressed data is strictly block-aligned via
fixed-sized output compression.

However, EROFS now supports big pclusters up to 1MiB and many users use
large configurations to minimize image sizes.  For such configurations,
the total number of extents decreases significantly (e.g., only 1,024
extents for a 1GiB file using 1MiB pclusters), then runtime metadata
overhead becomes negligible compared to data I/O and decoding costs.

Additionally, some popular compression algorithm (mainly Zstd) still
lacks native fixed-sized output compression support (although it's
planned by their authors).  Instead of just waiting for compressor
improvements, let's adopt byte-oriented extents, allowing these
compressors to retain their current methods.

For example, it speeds up Zstd compression a lot:
Processor: Intel(R) Xeon(R) Platinum 8163 CPU @ 2.50GHz * 96
Dataset:   enwik9
Build time Size       Type Command Line
3m52.339s  266653696  FO   -C524288 -zzstd,22
3m48.549s  266174464  FO   -E48bit -C524288 -zzstd,22
0m12.821s  272134144  FI   -E48bit -C1048576 --max-extent-bytes=1048576 -zzstd,22

0m14.528s  248987648  FO   -C1048576 -zlzma,9
0m14.605s  248504320  FO   -E48bit -C1048576 -zlzma,9

Encoded extents are structured as an array of `struct z_erofs_extent`,
sorted by logical address in ascending order:
   __le32 plen       // encoded length, algorithm id and flags
   __le32 pstart_lo  // physical offset LSB
   __le32 pstart_hi  // physical offset MSB
   __le32 lstart_lo  // logical offset
   __le32 lstart_hi  // logical offset MSB
   ..

Note that prefixed reduced records can be used to minimize metadata for
specific cases (e.g. lstart less than 32 bits, then 32 to 16 bytes).

If the logical lengths of all encoded extents are the same, 4-byte
(plen) and 8-byte (plen, pstart_lo) records can be used. Or, 16-byte
(plen .. lstart_lo) and 32-byte full records have to be used instead.

If 16-byte and 32-byte records are used, the total number of extents
is kept in `struct z_erofs_map_header`, and binary search can be
applied on them.  Note that `eytzinger order` is not considerd because
data sequential access is important.

If 4-byte records are used, 8-byte start physical offset is between
`struct z_erofs_map_header` and the `plen` array.

In addition, 64-bit physical offsets can be applied with new encoded
extent format to match full 48-bit block addressing.

Remove redundant comments around `struct z_erofs_lcluster_index` too.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Acked-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20250310095459.2620647-8-hsiangkao@linux.alibaba.com