aio_md_test: NUL-terminate result of readlink

readlink does not NUL-terminate the output buffer. This led to spurious
failures to destroy the md device because the unit number was garbage.
NUL-terminate the output buffer.

Reported by: ASLR
MFC after: 1 week
Sponsored by: Dell EMC Isilon

(cherry picked from commit ea0e1b19f232331bffa75a0a64a6859c40f1cc4d)