[PKT_SCHED]: Report rate estimator configuration errors during qdisc allocation

Current behaviour is to not report an error if a rate
estimator is created together with a qdisc and the
configuration of the rate estimator is bogus. This leads
to unexpected behaviour because the user is not notified.

New behaviour is to report the error and let the whole
qdisc creation operation fail so the user is able to fix
his mistake.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>