kernel - Correct vm_map_entry_reserve() call depth issue w/vkernel

kernel - Correct vm_map_entry_reserve() call depth issue w/vkernel

* vm_map was making vaddr tests against VM_MAX_USER_ADDRESS to
  determine when to call vm_map_entry_reserve().  This test works
  fine on real hardware but will not work properly for a vkernel.

* Instead, just use td_nest_count to prevent excessive recursion,
  or test against &kernel_map when necessary.

* Fixes SIGILL in vkernel due to stack overflow.

Reported-by: tuxillo