mac80211: clean up sta_info_destroy() users wrt. RCU/locking
Calling sta_info_destroy() doesn't require RCU-synchronisation
before-hand because it does that internally. However, it does
require rtnl-locking so insert that where necessary.
Also clean up the code doing it internally to be a bit clearer and
not synchronize twice if keys are configured.
Signed-off-by: Johannes Berg <johannes@sipsolutions.net> Signed-off-by: John W. Linville <linville@tuxdriver.com>