LU-4018 tests: improve racer file_create workload

LU-4018 tests: improve racer file_create workload

Use larger writes and smaller files, so that the racer file_create.sh
workload does not get bogged down with many threads stuck on sub-page
writes to the same file (causing excruciating 1kB-per-RPC sync writes
to disk on the OST).

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I173c155dfe4044f89deb8f2d8a7c3be4a28ce763
Reviewed-on: http://review.whamcloud.com/7951
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>