rgw: fix olh guarding scheme

is_olh() will return false if object is not yet an olh, although we
started the process into turning it into one. We should use the
existence of the olh tag instead.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
rgw: only retry on ECANCELED before doing the operation

After we did the operation (e.g., link olh, remove) we don't need to
retry. We got ECANCELED, therefore the operation has already been
applied by another user.

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