mac80211_hwsim: fix race in radio destruction from netlink notifier

The asynchronous destruction from a work-queue of radios tagged with
destroy-on-close may race with the owning namespace about to exit,
resulting in potential use-after-free of that namespace.

Instead of using a work-queue, move radios about to destroy to a
temporary list, which can be worked on synchronously after releasing
the lock. This should be safe to do from the netlink socket notifier,
as the namespace is guaranteed to not get released.

Signed-off-by: Martin Willi <martin@strongswan.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>