b43: Fix locking FIXME in beacon update top half

[Kalle Valo]

> b43 has a FIXME about locking in the mac80211 set-beacon-int callback for a long time.
> As it turns out there actually is a tiny race window that could result in
> a use-after-free bug of the 'current_beacon' memory.
> Nobody ever reported this, so it probably never happened.
> 
> Fix this by adding a spin lock that protects the current_beacon access.
> We must not be in atomic context while accessing hardware (due to SDIO),
> so the beacon update bottom half has to clone the skb and release the lock
> before writing it to hardware.
> 
> Let's all hope that this stops the troll who is trying to submit incorrect
> fixes for this issue repeatedly.
> And let's hope that I'm not a troll, too, who just hides even more evil code
> in an even more complex attempt to fix the issue.
> 
> Signed-off-by: Michael Buesch <m at bues.ch>
> Tested-by: Larry Finger <Larry.Finger at lwfinger.net>

Thanks, applied to wireless-drivers-next.git.

Kalle Valo