[NET_SCHED]: sch_htb: perform qlen adjustment immediately in ->delete

qlen adjustment should happen immediately in ->delete and not in the
class destroy function because the reference count will not hit zero in
->delete (sch_api holds a reference) but in ->put. Since the qdisc
lock is released between deletion of the class and final destruction
this creates an externally visible error in the qlen counter.

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