[3.12] gh-108111: Flush gzip write buffer before seeking, fixing bad writes (GH-108341) (#108402)

gh-108111: Flush gzip write buffer before seeking, fixing bad writes (GH-108341)
(cherry picked from commit 2eb60c1934f47671e6b3c9b90b6d9f1912d829a0)

Co-authored-by: Chris Markiewicz <effigies@gmail.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>