The ornamental design for a dinner fork, as shown and described.
FIG. 1 Front/top view of the dinner fork; and,
FIG. 2 Front/top perspective view of the dinner fork.
The Broken lines shown in the drawling are for illustrative purposes only and form no part of the claimed design. The design is the shape and configuration of the dinner fork shown in the solid lines. The most significant feature is the bridge/platform located in the tine area shown in solid lines of the drawing.