SCORU/Node/Injector: fix issue in reorg computation

Context

The injector would always show "Cannot compute reorg for" because of an off-by-one error in the number of predecessor we requested. This MR fixes this issue (the regression was introduced by !8618 (merged)).

The issue has only a limited impact on the injector but could make it forget operations if there are reorganizations which don't include the same operations.

Manually testing the MR

Running any Tezt test for the smart rollup node would show the following in the logs:

Cannot compute reorg for new block B...

This message should no appear any more.