pkg/tarsum: actually init the TarSum struct

closes #9241
ping @jlhawn and @stevvooe 

```
benchmark                         old ns/op     new ns/op     delta
Benchmark9kTar                    45186         46167         +2.17%
Benchmark9kTarGzip                1081241       1024691       -5.23%
Benchmark1mbSingleFileTar         21924444      21059648      -3.94%
Benchmark1mbSingleFileTarGzip     62401725      62360958      -0.07%
Benchmark1kFilesTar               97972656      97298579      -0.69%
Benchmark1kFilesTarGzip           227488476     223579873     -1.72%

benchmark                         old MB/s     new MB/s     speedup
Benchmark9kTar                    203.95       199.62       0.98x
Benchmark9kTarGzip                8.52         8.99         1.06x
Benchmark1mbSingleFileTar         47.83        49.79        1.04x
Benchmark1mbSingleFileTarGzip     16.80        16.81        1.00x
Benchmark1kFilesTar               10.70        10.78        1.01x
Benchmark1kFilesTarGzip           4.61         4.69         1.02x

benchmark                         old allocs     new allocs     delta
Benchmark9kTar                    22             22             +0.00%
Benchmark9kTarGzip                74             74             +0.00%
Benchmark1mbSingleFileTar         345            345            +0.00%
Benchmark1mbSingleFileTarGzip     3368           3371           +0.09%
Benchmark1kFilesTar               71123          71124          +0.00%
Benchmark1kFilesTarGzip           141169         141392         +0.16%

benchmark                         old bytes     new bytes     delta
Benchmark9kTar                    14201         14203         +0.01%
Benchmark9kTarGzip                1470322       1470322       +0.00%
Benchmark1mbSingleFileTar         41396         41363         -0.08%
Benchmark1mbSingleFileTarGzip     1906007       1906097       +0.00%
Benchmark1kFilesTar               2188442       2188440       -0.00%
Benchmark1kFilesTarGzip           10945384      10951599      +0.06%
```

Signed-off-by: Vincent Batts <vbatts@redhat.com>