Add new lock for stageq (part of ieee80211_superg structure) and ni_tx_superg (part of ieee80211_node structure); drop com_lock protection where it is used.
While here, drop duplicate OPACKETS counter incrementation.
ni_tx_ampdu is not protected with it (however, it is also used without locking in other places; probably, it requires some other solution to be thread-safe).
Since this change breaks KBI, it will not be merged into 11.0-RELEASE.