The ornamental design for a padlock, as shown and described.
FIG. 1 is a top and front perspective view of a padlock showing my new design;
FIG. 2 is a bottom and front perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a left side elevational view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken lines illustrate portions of the padlock that form no part of the claimed design. The appearance of any portion of the article between the break lines forms no part of the claimed design.