While the most precise naming might be "a singly linked list representation of a stack", "free list" (https://en.wikipedia.org/wiki/Free_list) is an ancient term of art for this problem.. so much so that wikipedia (at least the version today) even suggests "freelist" all one word as a spelling.

The initial linking together of all free space (also cache friendly, btw) is often called "threading the free list", although this terminology is less standard than "free list" itself.