rgw: fix vector index warning

rgw: fix vector index warning

src/rgw/rgw_etag_verifier.cc: In member function ‘virtual int RGWPutObj_ETagVerifier_MPU::process(ceph::bufferlist&&, uint64_t)’:
src/rgw/rgw_etag_verifier.cc:58:23: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::vector<long unsigned int>::size_type
   58 |   if (next_part_index == part_ofs.size()) {
      |       ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
src/rgw/rgw_etag_verifier.cc:76:25: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::vector<long unsigned int>::size_type
   76 |     if (next_part_index == part_ofs.size())
      |         ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~

Signed-off-by: Changcheng Liu <changcheng.liu@aliyun.com>