The ornamental design for a lavatory, as shown and described.
FIG. 1 is a perspective view of the lavatory embodying my new design;
FIG. 2 is a front elevational view thereof, the rear elevational view and left and right side elevational views appearing the same as the front elevational view;
FIG. 3 is a top plan view thereof; and,
FIG. 4 is a bottom plan view thereof.
The broken line representations in FIG. 4 are by way of illustration only and form no part of the claimed design.