erofs: support unaligned encoded data

erofs: support unaligned encoded data

We're almost there.  It's straight-forward to adapt the current
decompression subsystem to support unaligned encoded (compressed) data.

Note that unaligned data is not encouraged because of worse I/O and
caching efficiency unless the corresponding compressor doesn't support
fixed-sized output compression natively like Zstd.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Acked-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20250310095459.2620647-10-hsiangkao@linux.alibaba.com