The ornamental design for a mask, as shown and described.
FIG. 1 is a front isometric view of a mask;
FIG. 2 is a rear isometric view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a left side view thereof;
FIG. 7 is a right side view thereof; and,
FIG. 8 is a rear view thereof.
The broken lines illustrate a strap represent an environment and form no part of the claimed design, and the remaining broken lines illustrate portions of the mask that form no part of the claimed design.