mac80211: protect rx-path with spinlock

This patch fixes the problem which was discussed in
"mac80211: Fix PN corruption in case of multiple
virtual interface" [1].

Amit Shakya reported a serious issue with my patch:
mac80211: serialize rx path workers" [2]:

In case, ieee80211_rx_handlers processing is going on
for skbs received on one vif and at the same time, rx
aggregation reorder timer expires on another vif then
sta_rx_agg_reorder_timer_expired is invoked and it will
push skbs into the single queue (local->rx_skb_queue).

ieee80211_rx_handlers in the while loop assumes that
the skbs are for the same sdata and sta. This assumption
doesn't hold good in this scenario and the PN gets
corrupted by PN received in other vif's skb, causing
traffic to stop due to PN mismatch."

[1] Message-Id: http://mid.gmane.org/201302041844.44436.chunkeey@googlemail.com


[2] Commit-Id: 24a8fdad

Reported-by: Amit Shakya <amit.shakya@stericsson.com>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>