During compaction (`fill_partial_checksum_blocks`), when a partially overwritten
checksum block is encountered, the old data needs to be read from the base layer
(`clean_wr`). A coalesced slice is inserted into `read_vec` to logically reference
this base block data.
Previously, this coalesced slice was marked with `COPY_BUF_JOURNAL`. This is a
misuse of the flag because the underlying data originates from the data area (a
BIG_WRITE block), not the journal area. It should conceptually use `COPY_BUF_DATA`.
Although this misuse did not cause any functional bugs—because the coalesced slice
also carries the `COPY_BUF_COALESCED` flag, which perfectly short-circuits the I/O
submission in `read_buffered` and avoids erroneously reading from `journal_fd`—it
creates semantic confusion for future maintainers. `COPY_BUF_JOURNAL` is consistently
used elsewhere to denote small writes (journal data).
This commit changes the flag to `COPY_BUF_DATA` to ensure semantic correctness and
code self-consistency.
