mac80211_hwsim: fix use-after-free bug in hwsim_exit_net

When destroying a net namespace, all hwsim interfaces, which are not
created in default namespace are deleted. But the async deletion of the
interfaces could last longer than the actual destruction of the
namespace, which results to an use after free bug. Therefore use
synchronous deletion in this case.

Fixes: 100cb9ff ("mac80211_hwsim: Allow managing radios from non-initial namespaces")
Reported-by:  <syzbot+70ce058e01259de7bb1d@syzkaller.appspotmail.com>
Signed-off-by: Benjamin Beichler <benjamin.beichler@uni-rostock.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>