aio_md_test: label the md

Add a label to md devices created by this test. The next time this
test leaks md devices, finding the culprit will be much easier.

Thanks to: sobomax, for adding labels in r322969
MFC after: 1 week
Sponsored by: Dell EMC Isilon

(cherry picked from commit 9666cda9764a1d19cbf967940508dc6d2020cbfb)