Migration for questions older than 60 days is disabled network wide (even for moderators), and that makes any large scale migration more trouble than it's worth.
Furthermore out of 183 fstab questions, only 12 are closed, the Ask Ubuntu community doesn't seem to agree with your assertion that the questions don't belong here. I'm not saying they aren't on topic on Unix & Linux as well, but the second rule of migration is be a bit jealous of your site:
Respecting your own community
As members of a community, your first loyalty should be to that community. When evaluating a question, you shouldn’t be looking to push it off on some other site; instead, ask if it could be appropriate and on-topic for you, the experts who the author decided to ask. Be a bit jealous of your site – don’t blithely turn askers away simply because their question could be asked somewhere else. Don’t hit them over the head with your scope, help them tailor their question to fit into it – and if that means your site’s scope overlaps a bit with another site’s, so be it.
Obviously, there are questions you’ll have to turn away, either because their only connection to your site is via the audience (“How do I make bread as a programmer?”), because it’s completely off-topic (“How do I cook a fish in a dishwasher?” obviously belongs on Cooking, not Home Improvement) or because they’re simply not useful or constructive. But that should be your last resort. Close questions with an eye toward improvement and re-opening, not driving users away.
Since the AskUbuntu community in general seems to welcome the questions, no migration is necessary. They might not be Ubuntu specific, but they get expertly answered on AskUbuntu, and that's what matters most.