pkt_sched: Fix sch_sfq vs tc_modify_qdisc oops

sch_sfq as a classful qdisc needs the .leaf handler. Otherwise, there
is an oops possible in tc_modify_qdisc()/check_loop().

Fixes commit 7d2681a6

Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>