MRP-3604 ldlm: Wrong evict during failover

There is a race between setting obd_fail & OBD_OPT_FAILOVER.
tgt_client_del() checks only OBD_OPT_FAILOVER

It is better to rely only on obd_fail.
We shouldn't evict during failover at all, it should
be handled on a new server.
Such wrong evict can happen when server can't send CP AST
to the client because failover has started already.