There will be 21 year olds who are in college who want to smoke. Now, at my school you could legally drink in your dorm if you're 21 or you can go to a bar. But where will a 21-year-old college student in a dorm go to legally smoke assuming there is no bar in the area? Smoke outside? would they allow that? or would they just have to hide it like smoke in a car or something?
As for smoking in university housing, this will all depend on that particular college's policies regarding smoking marijuana in a dorm room. Most colleges probably currently have a general ban illegal substances, or they may have singled out marijuana in the case that students have MM cards and whether they allow cardholders to smoke in university housing. The university would then need to amend their housing rules/policies to allow smoking of marijuana in a dorm room. I see most colleges adopting/apply their smoking tobacco policies when marijuana is legal.
As I always say, if odor is an issue, vaporize. If I were in college today I'd go in on a volcano with friends and never combust, ever again.