sparsebit_next_clear
sparsebit_idx_t sparsebit_next_clear(const struct sparsebit *sbit, sparsebit_idx_t prev);
range_end = sparsebit_next_clear(s, range_begin) - 1; \
range_end = sparsebit_next_clear(s, range_begin) - 1)
idx = sparsebit_next_clear(s, idx);
for (idx = sparsebit_next_clear(s, start);
idx = sparsebit_next_clear(s, idx)) {
next = sparsebit_next_clear(s, first);
next = sparsebit_next_clear(s, first);
next = sparsebit_next_clear(s, first - 1);
next_cleared = sparsebit_next_clear(s, idx);
i = sparsebit_next_clear(region->unused_phy_pages, i);