t/zbd: Combine write and read fio commands for test case #6

t/zbd: Combine write and read fio commands for test case #6

The test case #6 checks result of the read job which reads data written
by the write job of test case #5. It does not work if test case #6 is
executed alone with the -t option and the target device has all zones
empty.

Specify both the write job and the read job to a single fio command
using write_and_run_one_fio_job() helper function instead of
run_fio_on_seq() helper function.

Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>