Do not allocate metadata via non-auto arenas, nor tcaches.

This assures that all internally allocated metadata come from the
first opt_narenas arenas, i.e. the automatically multiplexed arenas.

include/jemalloc/internal/tsd.h

@@ -536,6 +536,7 @@ struct tsd_init_head_s {
     O(thread_allocated,		uint64_t)				\
     O(thread_deallocated,	uint64_t)				\
     O(prof_tdata,		prof_tdata_t *)				\
+    O(iarena,			arena_t *)				\
     O(arena,			arena_t *)				\
     O(arenas_tdata,		arena_tdata_t *)			\
     O(narenas_tdata,		unsigned)				\
@@ -552,6 +553,7 @@ struct tsd_init_head_s {
     NULL,								\
     NULL,								\
     NULL,								\
+    NULL,								\
     0,									\
     false,								\
     tcache_enabled_default,						\