PARIS—Tunisian Ons Jabeur became the first major casualty on the opening day of the French Open on Sunday when the in-form sixth seed crashed out with a 3–6 7–6(4) 7–5 loss to Poland’s Magda Linette. Jabeur, seen as one of the big threats to world number one Iga Swiatek at the claycourt Grand Slam, came…