Propagate 64-bit unsigned object length to dump_content_length.

Of the callers, the s3 and swift backends already called with
a uint64_t total_len when an object is fetched. One caller
passes a const value of 0. The remaining three callers pass
the integer size of a formatted in-memory object.

Signed-off-by: Jan Harkes <jaharkes@cs.cmu.edu>