[benchmarks] Speed up the block IO performance test (biotime)

Pass an option to biotime to reduce the time it takes.  Without
passing this option, biotime reads the entire 111GB device, which is
excessive and takes 3.9 minutes!

Rename the "-tt" option to "-total-bytes-to-transfer" to make the code
more self-documenting.

Bug: PT-142
Test: garnet-x64-perf-dawson_canyon trybot
  + check time taken shown in bot log
Change-Id: I5f4b78179215e88a20dd914c832dcc77c37eb657