wifi: cfg80211: get the wiphy out of a dying network namespace

In the Linux kernel, the following vulnerability has been resolved:

wifi: cfg80211: get the wiphy out of a dying network namespace

When a network namespace is destroyed, cfg80211_pernet_exit() moves any
wiphy back to the initial namespace, and just warns if that fails. But
moving an interface can fail (due to allocation failures), and then the
wiphy is left behind with a garbage netns pointer:

Kernel mode fault at addr 0x30
genlmsg_multicast_netns.constprop.0+0x46/0xcf [cfg80211]
nl80211_notify_wiphy+0xcd/0xe8 [cfg80211]
wiphy_unregister+0x169/0x3fc [cfg80211]

Note that commit debac3a20dec ("net: Remove conflicting altnames for
dying netns in __dev_change_net_namespace().") fixed another path
that could reach it without allocation failures.

Remove interfaces that cannot be moved instead of failing the switch,
so that the wiphy always ends up in the initial namespace. In this
case the netdev core will unregister the interfaces anyway.