pkt_sched: Remove useless qdisc_stab_lock

The qdisc_stab_lock is used in qdisc_get_stab and qdisc_put_stab.
These two functions are invoked in qdisc_create, qdisc_change, and
qdisc_destroy which run fully under RTNL.

So it already makes sure only one could access the qdisc_stab_list at
the same time. Then it is unnecessary to use qdisc_stab_lock now.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>