t/zbd: fix fio failure check and SG node failure in test case 31

t/zbd: fix fio failure check and SG node failure in test case 31

The test case 31 runs fio twice but the failure of the first fio run was
not checked. This allowed the test case pass even with wrong
max_open_zones value. To fix this, check exit code of the fio run.

Also, the first fio run fails when the test target devices are SG nodes,
since libzbc I/O engine is not used. To fix this, call the ioengine()
helper function which adjusts I/O engine for each device.

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Link: https://lore.kernel.org/r/20230719105756.553146-11-shinichiro.kawasaki@wdc.com
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>