Before this change, blobfs-bench takes 5.3 minutes.  It covers 3*4=12
combinations of blob size and blob count.  The largest parameter
combination takes 3.7 minutes.  Skipping that combination reduces
blobfs-bench's overall runing time to 1.6 minutes.

Bug: PT-142
Test: garnet-x64-perf-dawson_canyon trybot
  + check time taken shown in bot log
Change-Id: I87845833708aa9100ead4ae2b22390be89a58c2e