Fix arenas_cache_cleanup().

Fix arenas_cache_cleanup() to handle allocation/deallocation within the
application's thread-specific data cleanup functions even after
arenas_cache is torn down.

test/unit/tsd.c

@@ -56,9 +56,14 @@ static void *
 thd_start(void *arg)
 {
 	data_t d = (data_t)(uintptr_t)arg;
+	void *p;
+
 	assert_x_eq(*data_tsd_get(), DATA_INIT,
 	    "Initial tsd get should return initialization value");
 
+	p = malloc(1);
+	assert_ptr_not_null(p, "Unexpected malloc() failure");
+
 	data_tsd_set(&d);
 	assert_x_eq(*data_tsd_get(), d,
 	    "After tsd set, tsd get should return value that was set");
@@ -67,6 +72,7 @@ thd_start(void *arg)
 	assert_x_eq(*data_tsd_get(), (data_t)(uintptr_t)arg,
 	    "Resetting local data should have no effect on tsd");
 
+	free(p);
 	return (NULL);
 }