t/zbd: Fix test target size of test case #48

t/zbd: Fix test target size of test case #48

Option --size was not specified to the fio command of test case #48. It
resulted in write operations to all available sequential write required
zones and relaxed zone locking test condition. Specify the option to
limit test target to 16 zones so that zone locking is tested with
expected condition.

Fixes: 3bd2078bdd1c ("zbd: add test for stressing zone locking")
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>