obj_bencher: allocate contentsChars to object_size, not op_size

Otherwise, our attempt to sanitize object_size bytes of
data.object_contents will be doomed to memory corruption.

Fixes: #7610
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1394 from ceph/wip-7610

obj_bencher: allocate contentsChars to object_size, not op_size

Reviewed-by: Sage Weil <sage@inktank.com>