wifi: mac80211: do not abuse fq.lock in ieee80211_do_stop()

lockdep complains use of uninitialized spinlock at ieee80211_do_stop() [1],
for commit f856373e ("wifi: mac80211: do not wake queues on a vif
that is being stopped") guards clear_bit() using fq.lock even before
fq_init() from ieee80211_txq_setup_flows() initializes this spinlock.

According to discussion [2], Toke was not happy with expanding usage of
fq.lock. Since __ieee80211_wake_txqs() is called under RCU read lock, we
can instead use synchronize_rcu() for flushing ieee80211_wake_txqs().

Link: https://syzkaller.appspot.com/bug?extid=eceab52db7c4b961e9d6 [1]
Link: https://lkml.kernel.org/r/874k0zowh2.fsf@toke.dk [2]
Reported-by: syzbot <syzbot+eceab52db7c4b961e9d6@syzkaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Fixes: f856373e ("wifi: mac80211: do not wake queues on a vif that is being stopped")
Tested-by: syzbot <syzbot+eceab52db7c4b961e9d6@syzkaller.appspotmail.com>
Acked-by: Toke Høiland-Jørgensen <toke@kernel.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/9cc9b81d-75a3-3925-b612-9d0ad3cab82b@I-love.SAKURA.ne.jp

net/mac80211/iface.c

@@ -590,9 +590,8 @@ static void ieee80211_do_stop(struct ieee80211_sub_if_data *sdata, bool going_do
 	bool cancel_scan;
 	struct cfg80211_nan_func *func;
 
-	spin_lock_bh(&local->fq.lock);
 	clear_bit(SDATA_STATE_RUNNING, &sdata->state);
-	spin_unlock_bh(&local->fq.lock);
+	synchronize_rcu(); /* flush _ieee80211_wake_txqs() */
 
 	cancel_scan = rcu_access_pointer(local->scan_sdata) == sdata;
 	if (cancel_scan)