mac80211: fix race between next beacon dtim and ieee80211_get_buffered_bc

mac80211: fix race between next beacon dtim and ieee80211_get_buffered_bc

On review of 'zd1211rw: implement beacon fetching and handling
ieee80211_get_buffered_bc()', Christian Lamparter noted that [1]:

   Since zd_beacon_done also uploads the next beacon so long in advance,
   there could be an equally long race between the outdated state of the
   next beacon's DTIM broadcast traffic indicator (802.11-2007 7.3.2.6)
   which -in your case- was uploaded almost a beacon interval ago and
   the xmit of ieee80211_get_buffered_bc *now*.

   The dtim bc/mc bit might be not set, when a mc/bc arrived after the
   beacon was uploaded, but before the "beacon done event" from the
   hardware. So, dozing stations don't expect the broadcast traffic
   and of course, they might miss it completely.

   It's probably better to fix this in mac80211 (see the attached hack).

[1] http://marc.info/?l=linux-wireless&m=129435041117256&w=2

CC: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>