kernel - Numerous VM MPSAFE fixes

kernel - Numerous VM MPSAFE fixes

* Remove most critical sections from the VM subsystem, these are no longer
  applicable (vm_token covers the access).

* _pmap_allocpte() for x86-64 - Conditionalize the zeroing of the vm_page
  after the grab.  The grab can race other threads and result in a page
  which had already been zero'd AND populated with pte's, so we can't just
  zero it.

  Use m->valid to determine if the page is actually newly allocated or not.

  NOTE: The 32 bit code already properly zeros the page by detecting whether
	the pte has already been entered or not.  The 64-bit code couldn't
	do this neatly so we used another method.

* Hold the pmap vm_object in pmap_release() and pmap_object_init_pt() for
  the x86-64 pmap code.  This prevents related loops from blocking on the
  pmap vm_object when freeing VM pages which is not expected by the code.

* pmap_copy() for x86-64 needs the vm_token, critical sections are no longer
  sufficient.

* Assert that PG_MANAGED is set when clearing pte's out of a pmap via the
  PV entries.  The pte's must exist in this case and it's a critical panic
  if they don't.

* pmap_replacevm() for x86-64 - Adjust newvm->vm_sysref prior to assigning
  it to p->p_vmspace to handle any potential MP races with other sysrefs
  on the vmspace.

* faultin() needs p->p_token, not proc_token.

* swapout_procs_callback() needs p->p_token.

* Deallocate the VM object associated with a vm_page after freeing the
  page instead of before freeing the page.  This fixes a potential
  use-after-refs-transition-to-0 case if a MP race occurs.