implement memory freeing for uring probe function

[glommer]

liburing now supports freeing memory allocated for usage with the feature discovery functionality.

I would be happy to provide the uring-sys implementation which I have in a private branch, but liburing would have to be updated to include at least:

axboe/liburing@caec738

[withoutboats]

Yea as I mentioned in #22, I'd like to pin to release commits of liburing. Fixing a bug I'd consider, but for new features I want to wait until axboe makes a release of liburing.