[NET_SCHED]: Fix filter double free

cbq and atm destroy their filters twice when destroying inner classes
during qdisc destruction.

Reported-and-tested-by: Strobl Anton <a.strobl@aws-it.at>

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>