zbd: Change sysfs path for partition devices

zbd: Change sysfs path for partition devices

zbd/001 and zbd/002 test cases fail for partition devices because of
sysfs path difference between partition devices and their holder
devices. The size parameter in sysfs path is different between the
partition devices and their holder devices. The holder devices have
nr_zones parameter in sysfs but the partition devices do not.

Utilize _test_dev_is_partition() helper function and TEST_DEV_PART_SYSFS
variable to refer correct sysfs size parameter for the partition devices.
Do not refer sysfs nr_zones parameter for the partition devices. Instead,
calculate the expected nr_zones from device capacity and zone size.

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>