kernel - Add workaround for improper yield in ACPI path

kernel - Add workaround for improper yield in ACPI path

* For now add a workaround for an improper yield that can occur indirectly
  via the ACPI path.  The problem is that the ACPI contrib code can hold
  a spinlock across a kmalloc() call.

* The ACPI code, in particular AcpiOsAcquireLock(), uses a spin lock.  At
  the same time it MUST use a spinlock because it might be called from
  the idle thread.  But it also appears that the code might call kmalloc()
  while holding a spinlock.

  The kmalloc path ACPI calls is with M_INTWAIT, which reduces the chance
  that kmalloc might try to block.  However, kmalloc is used to execute
  staged kfrees which can create a sequence:

  kmalloc -> kmem_slab_free -> (vm system) ->
  vm_object_page_remove_callback -> lwkt_user_yield().