mac80211: fix forwarded mesh frame queue mapping

We can't rely on ieee80211_select_queue() to do its job at this point
since the skb->protocol is not yet known. Instead, factor out and reuse
the queue mapping logic for injected frames.

Also, to mitigate congestion, forwarded frames should be dropped if the
outgoing queue was stopped.  This was not correctly implemented as we
were not checking the right queue.  Furthermore, we were dropping frames
that had arrived to their destination if that queue was stopped.

Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>