[PATCH v2 12/12] x86/traps: Fix up invalid PASID

[Luck, Tony]

> Are we planning to keep PASID live once a task has used it once or are we going to swap it lazily?  If the latter, a percpu variable might be better.

Current plan is "touch it once and the task owns it until exit(2)"

Maybe someday in the future when we have data on how applications
actually use accelerators we could look at something more complex
if usage patterns look like it would be beneficial.

-Tony