net_sched: let qdisc_put() accept NULL pointer

When tcf_block_get() fails in sfb_init(), q->qdisc is still a NULL
pointer which leads to a crash in sfb_destroy(). Similar for
sch_dsmark.

Instead of fixing each separately, Linus suggested to just accept
NULL pointer in qdisc_put(), which would make callers easier.

(For sch_dsmark, the bug probably exists long before commit
6529eab.)

Fixes: 6529eab ("net: sched: introduce tcf block infractructure")
Reported-by: syzbot+d5870a903591faaca4ae@syzkaller.appspotmail.com
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_generic.c

@@ -985,6 +985,9 @@ static void qdisc_destroy(struct Qdisc *qdisc)
 
 void qdisc_put(struct Qdisc *qdisc)
 {
+	if (!qdisc)
+		return;
+
 	if (qdisc->flags & TCQ_F_BUILTIN ||
 	    !refcount_dec_and_test(&qdisc->refcnt))
 		return;