test/nvmf: enable fio verify_backlog option

Using time_based tasks with do_verify=1 and rw=write
results in verification not being run. Basically
writes consume all run time and there's no time
left for verify:

fio: verification read phase will never start
because write phase uses all of runtime

Using verify_backlog we ask fio to verify after every
N blocks written.

Signed-off-by: Karol Latecki <karol.latecki@intel.com>
Change-Id: I4c1571f4fad893e47cc5fea2ad6b4e2a7d8cc5ef
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/11376


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Community-CI: Broadcom CI <spdk-ci.pdl@broadcom.com>
Reviewed-by: Pawel Piatek <pawelx.piatek@intel.com>
Reviewed-by: Konrad Sztyber <konrad.sztyber@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>