kernel - Rearrange usched_bsd4.c a bit

kernel - Rearrange usched_bsd4.c a bit

* Rearrange the way the scheduler helpers are woken up.  This results in
  much better coverage on systems with large numbers of cpus.

  Tested on the 48-core opteron monster.

* Essentially we no longer do bogus wakeups of scheduler helpers, and the
  chaining has been fixed when a scheduler helper is unable to perform
  the operation it was scheduled for (it tries to find another idle cpu
  to forward to).

  Most cpumask operations are now performed while holding the spin lock.