The ornamental design for a U-nut, as shown and described.
FIG. 1 is an isometric view of a front, top, and right side of an ornamental design for a U-nut;
FIG. 2 is a right side elevational view of the U-nut of FIG. 1;
FIG. 3 is a left side elevational view of the U-nut of FIG. 1;
FIG. 4 is a front elevational view of the U-nut of FIG. 1;
FIG. 5 is a rear elevational view of the U-nut of FIG. 1;
FIG. 6 is a top plan view of the U-nut of FIG. 1; and,
FIG. 7 is a bottom plan view of the U-nut of FIG. 1.
The dash-dash-dash lines are included for the purpose of illustrating portions that form no part of the claimed design. The dash-dot-dash lines showing boundary lines form no part of the claimed design.