The ornamental design for a facemask, as shown and described.
FIG. 1 is a perspective front view of a face mask in environmental conditions;
FIG. 2 shows a top view thereof;
FIG. 3 shows a front view thereof;
FIG. 4 shows a right side view thereof; and,
FIG. 5 shows a left side view thereof.
The broken lines immediately adjacent the shaded areas of the face mask represent the bounds of the claimed design while the broken lines outline of a person's head are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.