Limit maximum number of purged slabs with option

Option `experimental_hpa_max_purge_nhp` introduced for backward
compatibility reasons: to make it possible to have behaviour similar
to buggy `hpa_strict_min_purge_interval` implementation.

When `experimental_hpa_max_purge_nhp` is set to -1, there is no limit
to number of slabs we'll purge on each iteration. Otherwise, we'll purge
no more than `experimental_hpa_max_purge_nhp` hugepages (slabs). This in
turn means we might not purge enough dirty pages to satisfy
`hpa_dirty_mult` requirement.

Combination of `hpa_dirty_mult`, `experimental_hpa_max_purge_nhp` and
`hpa_strict_min_purge_interval` options allows us to have steady rate of
pages returned back to the system. This provides a strickier latency
guarantees as number of `madvise` calls is bounded (and hence number of
TLB shootdowns is limited) in exchange to weaker memory usage
guarantees.

test/unit/mallctl.c

@@ -292,6 +292,7 @@ TEST_BEGIN(test_mallctl_opt) {
 	TEST_MALLCTL_OPT(size_t, hpa_sec_max_bytes, always);
 	TEST_MALLCTL_OPT(size_t, hpa_sec_bytes_after_flush, always);
 	TEST_MALLCTL_OPT(size_t, hpa_sec_batch_fill_extra, always);
+	TEST_MALLCTL_OPT(ssize_t, experimental_hpa_max_purge_nhp, always);
 	TEST_MALLCTL_OPT(unsigned, narenas, always);
 	TEST_MALLCTL_OPT(const char *, percpu_arena, always);
 	TEST_MALLCTL_OPT(size_t, oversize_threshold, always);