t/zbd: prevent test #31 from looping

t/zbd: prevent test #31 from looping

The test 31 starts i/o to 128 zones in parallel.
There are two corner cases that are not properly handled in the
existing implementation -
1) If the total number of zones on the device is < 128, the test
will loop indefinitely because the loop increment is calculated as
zero by the script.
2) If the number of max_open_zones of the device is < 128, the
test will fail due to exceeding max_open_zones limit as the code
expects it to be >= 128.

Limit the number of open zones to the reported maximum
and skip the test if there is not enough zones on the device.

Signed-off-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Reviewed-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>