Add opt hpa_hugify_sync to hugify synchronously

Linux 6.1 introduced `MADV_COLLAPSE` flag to perform a best-effort
synchronous collapse of the native pages mapped by the memory range into
transparent huge pages.

Synchronous hugification might be beneficial for at least two reasons:
we are not relying on khugepaged anymore and get an instant feedback if
range wasn't hugified.

If `hpa_hugify_sync` option is on, we'll try to perform synchronously
collapse and if it wasn't successful, we'll fallback to asynchronous
behaviour.

src/jemalloc.c

@@ -1093,6 +1093,15 @@ validate_hpa_settings(void) {
 	if (opt_hpa_opts.dirty_mult != (fxp_t)-1 && validate_hpa_ratios()) {
 		had_conf_error = true;
 	}
+#ifndef JEMALLOC_HAVE_MADVISE_COLLAPSE
+	if (opt_hpa_opts.hugify_sync) {
+	       had_conf_error = true;
+	       malloc_printf(
+		   "<jemalloc>: hpa_hugify_sync config option is enabled, "
+		   "but MADV_COLLAPSE support was not detected at build "
+		   "time.");
+	}
+#endif
 }
 
 static void
@@ -1566,6 +1575,9 @@ malloc_conf_init_helper(sc_data_t *sc_data, unsigned bin_shard_sizes[SC_NBINS],
 			    0, 0, CONF_DONT_CHECK_MIN, CONF_DONT_CHECK_MAX,
 			    false);
 
+			CONF_HANDLE_BOOL(
+			    opt_hpa_opts.hugify_sync, "hpa_hugify_sync");
+
 			CONF_HANDLE_UINT64_T(
 			    opt_hpa_opts.min_purge_interval_ms,
 			    "hpa_min_purge_interval_ms", 0, 0,