wifi: mac80211: fix list iteration in ieee80211_add_virtual_monitor()

Since 'mon_list' of 'struct ieee80211_local' is RCU-protected and
an instances of 'struct ieee80211_sub_if_data' are linked there
via 'u.mntr.list' member, adjust the corresponding list iteration
in 'ieee80211_add_virtual_monitor()' accordingly.

Reported-by: [e-mail]
Closes: https://syzkaller.appspot.com/bug?extid=bc1aabf52d0a31e91f96
Fixes: a5aa46f1ac4f ("wifi: mac80211: track MU-MIMO configuration on disabled interfaces")
Signed-off-by: Dmitry Antipov <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Johannes Berg <[e-mail]>

net/mac80211/iface.c

···
 	if (!creator_sdata) {
 		struct ieee80211_sub_if_data *other;
 
-		list_for_each_entry(other, &local->mon_list, list) {
+		list_for_each_entry_rcu(other, &local->mon_list, u.mntr.list) {
 			if (!other->vif.bss_conf.mu_mimo_owner)
 				continue;