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GH-35665: [C++][Parquet] DeltaLengthByteArrayEncoder::Put reserve too much space (#35670)

### Rationale for this change

`BufferBuilder::Reserve` is different from `std::vector<T>::reserve`, its argument refers to additional bytes.

### What changes are included in this PR?

Avoid allocating too much memory space for the encoding sink buffer.

### Are these changes tested?

No. The change may affect runtime performance but not correctness.

### Are there any user-facing changes?

No.

* Closes: #35665

Lead-authored-by: mwish <maplewish117@gmail.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Signed-off-by: Antoine Pitrou <antoine@python.org>