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.

src/jemalloc.c

@@ -1558,6 +1558,10 @@ malloc_conf_init_helper(sc_data_t *sc_data, unsigned bin_shard_sizes[SC_NBINS],
 			    opt_hpa_opts.strict_min_purge_interval,
 			    "hpa_strict_min_purge_interval");
 
+			CONF_HANDLE_SSIZE_T(
+			    opt_hpa_opts.experimental_max_purge_nhp,
+			    "experimental_hpa_max_purge_nhp", -1, SSIZE_MAX);
+
 			if (CONF_MATCH("hpa_dirty_mult")) {
 				if (CONF_MATCH_VALUE("-1")) {
 					opt_hpa_opts.dirty_mult = (fxp_t)-1;