kernel - Major MPSAFE Infrastructure 2

kernel - Major MPSAFE Infrastructure 2

* Refactor buffer cache code which assumes content-stable data across
  a non-blocking BUF_LOCK().  This is no longer true.  The content must
  be reverified after the BUF_LOCK() succeeds.

* Make setting and clearing B_DELWRI atomic with buffer reassignment.

* Release cached mplock when looping in the scheduler and run
  check_splz() to avoid livelocking cpus.

* Refactor the mplock contention handling code to handle both the mplock
  and token contention.  Generate a 2uS delay for all but one cpu to
  try to avoid livelocks.

* Do not splz() from inside a spinlock, it will just panic.

* Fix the token description field for 'systat -pv 1'.

* Optimize MP_LOCK macros a bit.