rgw: keep track of written_objs correctly

rgw: keep track of written_objs correctly

Fixes: http://tracker.ceph.com/issues/15886
Only add a rados object to the written_objs list if the write
was successful. Otherwise if the write will be canceled for some
reason, we'd remove an object that we didn't write to. This was
a problem in a case where there's multiple writes that went to
the same part. The second writer should fail the write, since
we do an exclusive write. However, we added the object's name
to the written_objs list anyway, which was a real problem when
the old processor was disposed (as it was clearing the objects).

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>