wifi: mac80211: fix list iteration in ieee80211_add_virtual_monitor()

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: syzbot+bc1aabf52d0a31e91f96@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=bc1aabf52d0a31e91f96
Fixes: a5aa46f1ac4f ("wifi: mac80211: track MU-MIMO configuration on disabled interfaces")
Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Link: https://patch.msgid.link/20251204130533.340069-1-dmantipov@yandex.ru
Signed-off-by: Johannes Berg <johannes.berg@intel.com>