kv_flat_btree_async.cc: release AioCompletion before leave the loop

CID 727982 (#1 of 1): Resource leak (RESOURCE_LEAK)
  leaked_storage: Variable "aioc" going out of scope leaks the storage
  it points to.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
Merge pull request #297 from dalgaaf/wip-da-CID-727982

Reviewed-by: Sage Weil <sage@inktank.com>