write_mongodb: fix a couple of build warnings

write_mongodb: fix a couple of build warnings

  CC       src/write_mongodb_la-write_mongodb.lo
src/write_mongodb.c:96:21: warning: comparison of integers of different signs: 'int' and 'const size_t' (aka 'const unsigned long') [-Wsign-compare]
  for (int i = 0; i < ds->ds_num; i++) {
                  ~ ^ ~~~~~~~~~~
src/write_mongodb.c:121:21: warning: comparison of integers of different signs: 'int' and 'const size_t' (aka 'const unsigned long') [-Wsign-compare]
  for (int i = 0; i < ds->ds_num; i++) {
                  ~ ^ ~~~~~~~~~~
src/write_mongodb.c:134:21: warning: comparison of integers of different signs: 'int' and 'const size_t' (aka 'const unsigned long') [-Wsign-compare]
  for (int i = 0; i < ds->ds_num; i++) {
                  ~ ^ ~~~~~~~~~~
3 warnings generated.