The ornamental design for a chainsaw, as shown and described.
FIG. 1 is a left side perspective view of a chainsaw showing our new design;
FIG. 2 is a right ,side perspective view of a chainsaw showing our new design;
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a front view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken line showing of the cutting blade, guide bar and battery illustrates the environment of the claimed design and forms no part thereof. The remaining broken lines in the drawings illustrate portions of the chainsaw and form no part of the claimed design.