This fixes a blk file size calculation made during reindex that results in increased blk file malformity.
The fix is to avoid double counting the size of the serialization header during reindex.
This adds a unit test to reproduce the bug before the fix and to ensure that it does not recur.
These changes include a log message change also so as to not be as alarming. This is a common and recoverable
data corruption. These messages can now be filtered by the debug log reindex category.
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/24858 | core#24858]]