arrow

GH-41258: [C#][Integration] Fix comparison of sliced validity buffers with non-zero offsets (#41259)


### Rationale for this change

After https://github.com/apache/arrow/pull/41230 , the integration tests are failing on main .

### What changes are included in this PR?

The bit-by-bit comparison branch in the validity bitmap comparison is missing an offset on one side of the comparison. This PR adds that offset back in.

### Are these changes tested?

Via the integration test CI job.

### Are there any user-facing changes?

No
* GitHub Issue: #41258

Authored-by: Dewey Dunnington <dewey@fishandwhistle.net>
Signed-off-by: Dewey Dunnington <dewey@fishandwhistle.net>