Add SKB_CB macro and begin using it in a few places, eventually everywhere

Add SKB_CB macro and begin using it in a few places, eventually everywhere
   we use skb->cb (will be in a subsequent commit/merge) should use
   this macro.  This is part of a larger batch of changes I am in the process
   of merging that add better visibility into skbuff usage/leaks in the driver.

ieee80211_node_refcnt use atomic_read for consistency and because it is
   the correct thing to do.  atomic_read is a macro that expands to the same
   thing in most architectures anyway.

git-svn-id: http://madwifi-project.org/svn/madwifi/trunk@2887 0192ed92-7a03-0410-a25b-9323aeb14dbd