The ornamental design for a mattress, as shown and described.
1. Mattress
1.1 : Bottom perspective view
1.2 : Front
1.3 : Back
1.4 : Top
1.5 : Bottom
1.6 : Left
1.7 : Right
1.8 : Partial enlarged bottom view
1.9 : Sectional view
1.10 : Sectional view
The even dash broken lines show portions of the mattress that form no part of the claimed design. The long-dash, short-dash broken lines represent the boundaries of the claim and form no part thereof.
Reproduction 1.9 is an enlarged cross-sectional view taken along the center of reproduction 1.8. Reproduction 1.10 is an enlarged cross-sectional view a little bit above the center of reproduction 1.8 at the claimed four circular holes.