Cache bin: expose ncached_max publicly.

include/jemalloc/internal/cache_bin.h

@@ -20,6 +20,16 @@
  */
 typedef uint16_t cache_bin_sz_t;
 
+/*
+ * That implies the following value, for the maximum number of items in any
+ * individual bin.  The cache bins track their bounds looking just at the low
+ * bits of a pointer, compared against a cache_bin_sz_t.  So that's
+ *   1 << (sizeof(cache_bin_sz_t) * 8)
+ * bytes spread across pointer sized objects to get the maximum.
+ */
+#define CACHE_BIN_NCACHED_MAX (((size_t)1 << sizeof(cache_bin_sz_t) * 8) \
+    / sizeof(void *) - 1)
+
 /*
  * This lives inside the cache_bin (for locality reasons), and is initialized
  * alongside it, but is otherwise not modified by any cache bin operations.