Bypass extent tracking for auto arenas.

Tracking extents is required by arena_reset.  To support this, the extent
linkage was used for tracking 1) large allocations, and 2) full slabs.  However
modifying the extent linkage could be an expensive operation as it likely incurs
cache misses.  Since we forbid arena_reset on auto arenas, let's bypass the
linkage operations for auto arenas.

include/jemalloc/internal/jemalloc_internal_inlines_b.h

@@ -70,6 +70,12 @@ arena_ichoose(tsd_t *tsd, arena_t *arena) {
 	return arena_choose_impl(tsd, arena, true);
 }
 
+JEMALLOC_INLINE bool
+arena_is_auto(arena_t *arena) {
+	assert(narenas_auto > 0);
+	return (arena_ind_get(arena) < narenas_auto);
+}
+
 JEMALLOC_ALWAYS_INLINE extent_t *
 iealloc(tsdn_t *tsdn, const void *ptr) {
 	rtree_ctx_t rtree_ctx_fallback;