mac80211_hwsim: fix use-after-free bug in hwsim_exit_net

commit 8cfd36a0b53aeb4ec21d81eb79706697b84dfc3d upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>