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

lockdep complains use of uninitialized spinlock at ieee80211_do_stop() [1],
for commit f856373e2f31ffd3 ("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/[e-mail] [2]
Reported-by: syzbot <[e-mail]>
Signed-off-by: Tetsuo Handa <[e-mail]>
Fixes: f856373e2f31ffd3 ("wifi: mac80211: do not wake queues on a vif that is being stopped")
Tested-by: syzbot <[e-mail]>
Acked-by: Toke Høiland-Jørgensen <[e-mail]>
Signed-off-by: Kalle Valo <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

net/mac80211/iface.c

···
 	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)