mac80211: fix spinlock recursion

When STAs are expired, we need to hold the sta_lock. Using
the same lock for keys too would then mean we'd need another
key free function, and that'll just lead to confusion, so just
use a new spinlock for all key lists.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>