mac80211: fix RCU locking in __ieee80211_rx_handle_packet

Commit c7a51bda ("mac80211: restructure __ieee80211_rx") extracted
__ieee80211_rx_handle_packet out of __ieee80211_rx and hence changed
the locking rules for __ieee80211_rx_handle_packet(), it is now
invoked under RCU lock. There is, however, one instance left where
it contains an rcu_read_unlock() in an error path, which is a bug.

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