write_mongodb: fix build warning

write_mongodb: fix build warning

  CC       src/write_mongodb_la-write_mongodb.lo
src/write_mongodb.c: In function ‘wm_config’:
src/write_mongodb.c:371:55: warning: ‘%s’ directive output may be truncated writing up to 127 bytes into a region of size 114 [-Wformat-truncation=]
     snprintf(cb_name, sizeof(cb_name), "write_mongodb/%s", node->name);
                                                       ^~
src/write_mongodb.c:371:5: note: ‘snprintf’ output between 15 and 142 bytes into a destination of size 128
     snprintf(cb_name, sizeof(cb_name), "write_mongodb/%s", node->name);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~