wapbl_space_used
return avail - wapbl_space_used(avail, head, tail);
KASSERT(delta <= wapbl_space_used(size, head, tail));
wapbl_space_used(wl->wl_circ_size, head, tail));
wapbl_space_used(wl->wl_circ_size, head, tail),
wapbl_space_used(wl->wl_circ_size, head, tail),
static inline size_t wapbl_space_used(size_t, off_t, off_t);
used = wapbl_space_used(wch->wc_circ_size, wch->wc_head, wch->wc_tail);