t/zbd: avoid test case 31 failure with small devices

t/zbd: avoid test case 31 failure with small devices

The test case assumed that the test target devices have 128 or more
sequential write required zones and uses 128 as the minimum number of
zones to write. This caused failure when the devices had a smaller
number of sequential write required zones. To avoid the failure, count
the actual number of sequential write required zones and use it if it is
smaller than 128.

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Link: https://lore.kernel.org/r/20240430103022.4136039-4-shinichiro.kawasaki@wdc.com
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>