librbd/internal.cc: fix resource leak

Call release() on librados::AioCompletion to free storage before
leave the loop or call new again.

CID 1021213 (#1 of 1): Resource leak (RESOURCE_LEAK)
  leaked_storage: Variable "rados_completion" going out of scope leaks
  the storage it points to.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
Merge pull request #296 from dalgaaf/wip-da-CID-1021213

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>