net_sched: prevent NULL dereference if default qdisc setup failed

If qdisc_create_dflt() fails, it returns NULL. With CONFIG_NET_SCHED
enabled, the check qdisc != &noop_qdisc passes and qdisc will be passed
to qdisc_hash_add(), which dereferences it.

This assignment was present in the upstream commit 5891cd5ec46c2
("net_sched: add __rcu annotation to netdev->qdisc") but was missed in
the backport 22d95b54 ("net_sched: add __rcu annotation to
netdev->qdisc"), perhaps due to merge conflicts.  dev->qdisc is
&noop_qdisc by default and if qdisc_create_dflt() fails, this assignment
will make sure qdisc == &noop_qdisc and no NULL dereference will take
place.

This bug was discovered and resolved using Coverity Static Analysis
Security Testing (SAST) by Synopsys, Inc.

Fixes: 22d95b54 ("net_sched: add __rcu annotation to netdev->qdisc")
Signed-off-by: Pratyush Yadav <ptyadav@amazon.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>