Change kDefaultZeroCopyThreshold to 0 to avoid a regression and avoid a failure while...

Change kDefaultZeroCopyThreshold to 0 to avoid a regression and avoid a failure while running as not root

Summary:
Change kDefaultZeroCopyThreshold to 0 to avoid a regression when using a buffer chain that exceeds 32K but each buffer is small.
Change the benchmark to set it's own threshold. Also use calloc vs malloc (in the benchmark only) to get around some weird kernel interaction on non zero copy enabled systems - 2 back to back tests report very different results.

Reviewed By: djwatson

Differential Revision: D6112299

fbshipit-source-id: 3895d3ece2925c4626284ff364495708293edc3e