[PATCH] Fix tst-aio7.

[Roland McGrath]

> Then these variables have to have global scope.
> 
> The test in question was also testing that the AIO handling at exit time
> is leak-free.  There is not explicit test but one easily sees system
> resources being depleated.  This is important once we have a
> kernel-supported AIO implementation.

Ok.  I've reverted the aio_cancel change and instead given CB1 permanent
extent so that it's valid.  I put in a comment mentioning this rationale.