Virtual address allocator
struct vas_range { struct vas_range *next_free; struct rbt_node node; vaddr_t start; size_t length;};struct vas_range referenced types:
struct vas_space { struct spinlock lock; struct rbt tree; vaddr_t base; vaddr_t limit; struct vas_range *freelist; struct vas_set *percpu_sets;};struct vas_space referenced types: