vmem_roundup_size
const vmem_size_t size = vmem_roundup_size(vm, size0);
KASSERT(bt->bt_size == vmem_roundup_size(vm, size) ||
bt->bt_size - vmem_roundup_size(vm, size) <= vm->vm_quantum_mask);
vmem_size_t vmem_roundup_size(vmem_t *, vmem_size_t);