sparsebit_next_set
sparsebit_idx_t sparsebit_next_set(const struct sparsebit *sbit, sparsebit_idx_t prev);
range_begin = sparsebit_next_set(s, range_end), \
base = pg = sparsebit_next_set(region->unused_phy_pages, pg);
next_set = sparsebit_next_set(s, idx);
for (idx = sparsebit_next_set(s, start);
idx = sparsebit_next_set(s, idx)) {
idx = sparsebit_next_set(s, idx);
next = sparsebit_next_set(s, first);
next = sparsebit_next_set(s, first - 1);
next = sparsebit_next_set(s, first);