t/zbd: add missing prep_write for test cases with write workloads

t/zbd: add missing prep_write for test cases with write workloads

The test cases from 54 to 57 do writes but miss prep_write() call which
resets zones of the test target device with max_active_zones limit. This
results in failures due to open zones out of I/O ranges and
max_active_zones limit error. Add the missing prep_write() call to avoid
the failures.

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Link: https://lore.kernel.org/r/20230719105756.553146-12-shinichiro.kawasaki@wdc.com
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>