kernel - more lwbuf followup work

kernel - more lwbuf followup work

* Make lwbuf objcache only, removing all the manual per-cpu allocation
  tracking.  Keep the cpumask stuff.  We will deal with the KVM
  fragmentation issue inside objcache later on.

* This basically takes us back to Sam's original objcache implementation.

* Remove unnecessary assembly symbols (assembly didn't use those globaldata
  fields).  Remove related globaldata fields now that we are back to the
  objcache-only implementation.