wifi: mac80211: fix general-protection-fault in ieee80211_subif_start_xmit()

wifi: mac80211: fix general-protection-fault in ieee80211_subif_start_xmit()

When device is running and the interface status is changed, the gpf issue
is triggered. The problem triggering process is as follows:
Thread A:                           Thread B
ieee80211_runtime_change_iftype()   process_one_work()
    ...                                 ...
    ieee80211_do_stop()                 ...
    ...                                 ...
        sdata->bss = NULL               ...
        ...                             ieee80211_subif_start_xmit()
                                            ieee80211_multicast_to_unicast
                                    //!sdata->bss->multicast_to_unicast
                                      cause gpf issue

When the interface status is changed, the sending queue continues to send
packets. After the bss is set to NULL, the bss is accessed. As a result,
this causes a general-protection-fault issue.

The following is the stack information:
general protection fault, probably for non-canonical address
0xdffffc000000002f: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000178-0x000000000000017f]
Workqueue: mld mld_ifc_work
RIP: 0010:ieee80211_subif_start_xmit+0x25b/0x1310
Call Trace:
<TASK>
dev_hard_start_xmit+0x1be/0x990
__dev_queue_xmit+0x2c9a/0x3b60
ip6_finish_output2+0xf92/0x1520
ip6_finish_output+0x6af/0x11e0
ip6_output+0x1ed/0x540
mld_sendpack+0xa09/0xe70
mld_ifc_work+0x71c/0xdb0
process_one_work+0x9bf/0x1710
worker_thread+0x665/0x1080
kthread+0x2e4/0x3a0
ret_from_fork+0x1f/0x30
</TASK>

Fixes: 017adf1ffc6d ("wifi: mac80211: do not wake queues on a vif that is being stopped")
Reported-by: syzbot+c6e8fca81c294fd5620a@syzkaller.appspotmail.com
Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Link: https://lore.kernel.org/r/20221026063959.177813-1-shaozhengchao@huawei.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>