The ornamental design for a linear actuator shaft, as shown and described.
FIG. 1 is an end/side isometric view of a linear actuator shaft of my new design, with the shaft thereof in extended position;
FIG. 2 is an end/side isometric view thereof, with the shaft in retracted position;
FIG. 3 is a side elevational view thereof, with the shaft in extended position; and,
FIG. 4 is an end elevational view thereof; it being understood that the matter shown in broken lines on the drawing figures is for illustrative purposes only and forms no part of the claimed design.