rgw: warning, output may be truncated before the last format character

rgw: warning, output may be truncated before the last format character

The following warning appears during make:

```
rgw_rados.cc: In function ‘void usage_log_hash(CephContext*, const string&, std::__cxx11::string&, uint32_t)’:
rgw_rados.cc:4579:13: warning: ‘__builtin___snprintf_chk’ output may be truncated before the last format character [-Wformat-truncation=]
static void usage_log_hash(CephContext *cct, const string& name, string& hash, uint32_t index)
                 ^~~~~~~~~~~~~~
/usr/include/bits/stdio2.h:65:44: note: ‘__builtin___snprintf_chk’ output between 8 and 17 bytes into a destination of size 16
        __bos (__s), __fmt, __va_arg_pack ());
```

Signed-off-by: Jos Collin <jcollin@redhat.com>