rgw: fix can not disable max_size quota

Currently if we enable quota and set max_size = -1 trying to disable max size quota, it turns out to be max_size_kb = 0 and it will cause we can not write any object. The root cause is rgw_rounded_kb returns 0 for -1. We should not convert negative value to uint64_t. 

Sign off by: Dong Lei leidong@yahoo-inc.com