fellow cache: Use objsize_hint to estimate chunked size

Fixes the second part of #60

src/fellow_cache.c

@@ -5098,6 +5098,8 @@ fellow_busy_obj_getspace(struct fellow_busy *fbo, size_t *sz, uint8_t **ptr)
 	chunksize = (size_t)1 << fbo->fc->tune->chunk_exponent;
 	assert(chunksize <= FIO_MAX);
 
+	if (*sz == 0)
+		*sz = fbo->fc->tune->objsize_hint;
 	if (*sz == 0)
 		*sz = chunksize;