ceph_test_rados_api_*: fix build warnings, memset ranges

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1668 from ceph/wip-librados-tests

ceph_test_rados_api_*: fix build warnings and memset ranges

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>