nl80211: rework locking

When I added scanning to cfg80211, we got a lock dependency like this:
	rtnl --> cfg80211_mtx

nl80211, on the other hand, has the reverse lock dependency:
	cfg80211_mtx --> rtnl

which clearly is a bad idea. This patch reworks nl80211 to take these
two locks in the other order to fix the possible, and easily
triggerable, deadlock.

Signed-off-by: Johannes Berg <[email protected]>
Signed-off-by: John W. Linville <[email protected]>