they're usually a bit more speedy than trance and house rolls
Honestly, I think people use "roll" to describe the groove of a track - if I was going to call any specific element a roll, it would be the part of a break beat between the two snares. If I was going to call a bassline a roll, it would probably have an LFO on the pitch or filter, or both - and a downward gliss - think "Badass".
Calling DnB rollin is like calling funk funkin - it's when the groove sets in and it just sounds right. You know it's just a two bar loop but you could still listen for 5 minutes.