[PKT_SCHED]: Fix missing qdisc_destroy() in qdisc_create_dflt()

qdisc_create_dflt() is missing to destroy the newly allocated
default qdisc if the initialization fails resulting in leaks
of all kinds. The only caller in mainline which may trigger
this bug is sch_tbf.c in tbf_create_dflt_qdisc().

Note: qdisc_create_dflt() doesn't fulfill the official locking
      requirements of qdisc_destroy() but since the qdisc could
      never be seen by the outside world this doesn't matter
      and it can stay as-is until the locking of pkt_sched
      is cleaned up.

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