ceph_test_cls_hello: set RETURNVEC on the expected EINVAL request

ceph_test_cls_hello: set RETURNVEC on the expected EINVAL request

We are sending a request that returns an error *and* a data payload.  In order
to reliably get the payload, we need to set the RETURNVEC flag so that it is
recorded in the pg log.  Otherwise, we might get unlucky and resend the request
and end up in the dup handler and get the error code but no payload.

Fixes: https://tracker.ceph.com/issues/44385
Signed-off-by: Sage Weil <sage@redhat.com>