t/zbd: skip test case #13 when max_open_zones is too small

t/zbd: skip test case #13 when max_open_zones is too small

Test case #13 of t/zbd/test-zbd-support fails when the test target
device has max_open_zones smaller than 4. To avoid the failure, add a
helper function require_max_open_zones and use it to skip the test case
when the device does not have required minimum max_open_zones.

Reported-by: Vincent Fu <vincent.fu@samsung.com>
Link: https://lore.kernel.org/fio/20220602015316.6ismlexb22fwd5ko@shindev/
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Link: https://lore.kernel.org/r/20220602091310.97189-1-shinichiro.kawasaki@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>