Simplify counting of message bytes.

benchmark           old ns/op    new ns/op    delta
BenchmarkFull8            100          100   +0.00%
BenchmarkFull16           120          114   -5.00%
BenchmarkFull40           115          113   -1.74%
BenchmarkFull64           220          214   -2.73%
BenchmarkFull128          220          216   -1.82%
BenchmarkFull1K          2054         2007   -2.29%
BenchmarkFull8K         16102        15581   -3.24%

benchmark            old MB/s     new MB/s  speedup
BenchmarkFull8          79.73        79.41    1.00x
BenchmarkFull16        132.42       139.21    1.05x
BenchmarkFull40        208.42       210.75    1.01x
BenchmarkFull64        290.55       298.00    1.03x
BenchmarkFull128       579.46       592.46    1.02x
BenchmarkFull1K        498.53       510.12    1.02x
BenchmarkFull8K        508.73       525.76    1.03x