The ornamental design for a hairnet, as shown and described.
FIG. 1 is a front perspective view of a hair net according to my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is one side elevational view thereof;
FIG. 4 is a rear elevational view thereof; and,
FIG. 5 is another front elevation view thereof.
The broken lines of the partial human figure as well as the broken lines shown on the hair net represent environmental structure that form no part of the claimed design.