kernel - Fix SMP race in VMM/VMX vmclear() call

* Fix an SMP race.  The pcpu vti field is used to trigger cpusync's from
  other cpus and cannot be cleared by the owning cpu until after it has
  vmclear'd and stopped using the structure.

Reported-by: tuxillo
kernel - Try to catch possible bug in getnewbuf()

* Add a debugging kprintf() and cleanup code to attempt to catch a possible
  bug in getnewbuf() where a buffer is somehow left on a vnode's clean or
  dirty list before being reused.