The ornamental design for a rollerskate, as shown and described.
FIG. 1 is a top, right perspective view of the roller skate of my design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a right elevation view thereof;
FIG. 5 is a left elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines shown represent unclaimed subject matter and form no part of the claimed design. Specifically, the buckle and teeth shown in FIGS. 1-7 are for illustrative purposes and form no part of the claimed design.