mac80211: fix deadlock in AP/VLAN handling

Syzbot reports that when you have AP_VLAN interfaces that are up
and close the AP interface they belong to, we get a deadlock. No
surprise - since we dev_close() them with the wiphy mutex held,
which goes back into the netdev notifier in cfg80211 and tries to
acquire the wiphy mutex there.

To fix this, we need to do two things:
 1) prevent changing iftype while AP_VLANs are up, we can't
    easily fix this case since cfg80211 already calls us with
    the wiphy mutex held, but change_interface() is relatively
    rare in drivers anyway, so changing iftype isn't used much
    (and userspace has to fall back to down/change/up anyway)
 2) pull the dev_close() loop over VLANs out of the wiphy mutex
    section in the normal stop case

Cc: stable@vger.kernel.org
Reported-by:  <syzbot+452ea4fbbef700ff0a56@syzkaller.appspotmail.com>
Fixes: a05829a7 ("cfg80211: avoid holding the RTNL when calling the driver")
Link: https://lore.kernel.org/r/20210517160322.9b8f356c0222.I392cb0e2fa5a1a94cf2e637555d702c7e512c1ff@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>