cfg80211: fix race in netlink owner interface destruction

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

cfg80211: fix race in netlink owner interface destruction

My previous fix here to fix the deadlock left a race where

the exact same deadlock (see the original commit referenced

below) can still happen if cfg80211_destroy_ifaces() already

runs while nl80211_netlink_notify() is still marking some

interfaces as nl_owner_dead.

The race happens because we have two loops here - first we

dev_close() all the netdevs, and then we destroy them. If we

also have two netdevs (first one need only be a wdev though)

then we can find one during the first iteration, close it,

and go to the second iteration -- but then find two, and try

to destroy also the one we didn't close yet.

Fix this by only iterating once.