test/bdev: fix fio test verification

When this test was ran, it reported for each job:
"fio: verification read phase will never start because write phase uses all of runtime"

There was only randwrite phase, without any read phase with verification.
Alternatievly we can verify a set number of blocks during write using
"verify_backlog" argument.

With this patch writes to all bdevs will be verfied in 1024 chunks.

Change-Id: Ie79d543badd10f3b3ca91b3b56d9d8cb0dd332b2
Signed-off-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/464646


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Broadcom SPDK FC-NVMe CI <spdk-ci.pdl@broadcom.com>