kernel - Concurrent fork/exec (3) - Fix 32-bit builds & vkernels

* 32-bit kernels and both 32 and 64-bit vkernels were setting
  kernel_pmap.pm_pteobj to kernel_object.  This creates a shared/excl
  race that locks them up.

* Replace with kptobj, a vm_object dedicated to the kernel_pmap.

* Not applicable on normal 64-bit kernels as they use a more modern
  pmap implementation that does not require a pm_pteobj.

Reported-by: tuxillo