osd: avoid multi set osd_op.outdata in tier pool
There are two read op on the same object for ec pool. First op read
miss happend, calling do_proxy_read and promote_object, The second op only
do_proxy_read. but before first op process_copy_chunk finish, the second op
finish_proxy_read. first op receive reply from base pool
first and then second received. so the second op set the field "outdata"
in m->ops first. And then first op requeue_ops in process_copy_chunk,
At last in do_osd_ops, it append outdata field.