Fix stats.arenas.<i>.[...] for --disable-stats case.

Add missing stats.arenas.<i>.{dss,lg_dirty_mult,decay_time}
initialization.

Fix stats.arenas.<i>.{pactive,pdirty} to read under the protection of
the arena mutex.

include/jemalloc/internal/arena.h

@@ -572,7 +572,10 @@ ssize_t	arena_lg_dirty_mult_default_get(void);
 bool	arena_lg_dirty_mult_default_set(ssize_t lg_dirty_mult);
 ssize_t	arena_decay_time_default_get(void);
 bool	arena_decay_time_default_set(ssize_t decay_time);
-void	arena_stats_merge(arena_t *arena, const char **dss,
+void	arena_basic_stats_merge(arena_t *arena, unsigned *nthreads,
+    const char **dss, ssize_t *lg_dirty_mult, ssize_t *decay_time,
+    size_t *nactive, size_t *ndirty);
+void	arena_stats_merge(arena_t *arena, unsigned *nthreads, const char **dss,
     ssize_t *lg_dirty_mult, ssize_t *decay_time, size_t *nactive,
     size_t *ndirty, arena_stats_t *astats, malloc_bin_stats_t *bstats,
     malloc_large_stats_t *lstats, malloc_huge_stats_t *hstats);

include/jemalloc/internal/ctl.h

@@ -38,6 +38,9 @@ struct ctl_arena_stats_s {
 	ssize_t			decay_time;
 	size_t			pactive;
 	size_t			pdirty;
+
+	/* The remainder are only populated if config_stats is true. */
+
 	arena_stats_t		astats;
 
 	/* Aggregate stats for small size classes, based on bin stats. */

include/jemalloc/internal/private_symbols.txt

@@ -2,6 +2,7 @@ a0dalloc
 a0malloc
 arena_aalloc
 arena_alloc_junk_small
+arena_basic_stats_merge
 arena_bin_index
 arena_bin_info
 arena_bitselm_get