test/nvmf: add verify_backlog to fio SGL tests.

On newer versions of FIO, there is an issue with heavy verify workloads
where one of the headers (rand_seed) gets incorrectly generated by fio
during verify. This can be circumvented by using the verify_backlog
flag.

This is needed because it will enable testing this workload on the tcp
transport using fio in the SPDK test pool.

Change-Id: I028be3fdb72a76733b4226a37b6332cd45d0f774
Signed-off-by: Seth Howell <seth.howell@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/988


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>