save queue code.
Instead, use if_transmit() directly - and handle the cases where frame
transmission fails.
I don't necessarily like this and I think at this point the M_ENCAP check,
node freeing upon fail and the actual if_transmit() call should be done
in methods in ieee80211_freebsd.c, but I digress slightly..
This removes one of the last few uses of if_start() and the ifnet
if_snd queue. The last major offender is ieee80211_output.c, where
ieee80211_start() implements if_start() and uses the ifnet queue
directly.
(There's a couple of gotchas here, where the if_start pointer is
compared to ieee80211_start(), but that's a later problem.)
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