While it would be nice to simply make request_percpu_irq() take an affinity
mask, the churn is likely to be on the irritating side given that most
drivers do not give a damn about affinities.
So take the more innocuous path to provide a helper that parallels
request_percpu_irq(), with an affinity as a bonus argument.
Yes, request_percpu_irq_affinity() is a bit of a mouthful.