The ornamental design for a sling mount, as shown and described.
FIG. 1 is a perspective view of the sling mount;
FIG. 2 is a front view;
FIG. 3 is a back view;
FIG. 4 is a left side view;
FIG. 5 is a right side view;
FIG. 6 is a top view; and,
FIG. 7 is a bottom view.
The broken lines immediately adjacent the shaded areas represent the bounds of the claimed design while all other broken lines are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.