otus(4) - use the local node alloc function so there's space for statistics.

otus(4) - use the local node alloc function so there's space for statistics.

* Use the correct malloc type for node allocation - M_80211_NODE - so
  the default node free method in net80211 will work correctly.
* Fix otus_node_alloc() to suit FreeBSD's net80211.
* .. and actually call otus_node_alloc() so there's space for the
  per-node tx statistics.  Otherwise, well, it will be scribbling over
  random memory.

Tested:

* AR9170, STA mode