For rum(4) and ural(4):

For rum(4) and ural(4):
- Use crit section to protect external interfaces, callouts, USB
  task and USB operation callback functions.
- Avoid holding ifnet.if_serializer across various USB operations,
  since current USB stack does not aware of lwkt serializer.
- In various callout, USB task and USB operation callback functions,
  hold ifnet.if_serializer around ieee80211 functions.