fix forkexectree

hopefully this will migrate less state when stealing.

with tlb shootdowns disabled, radix scales linearly, and crange has
sub-linear scaling.

with VM_RADIX:

  $ time mapbench 1
  12834002141 cycles
  $ time mapbench 2
  12230873801 cycles
  $ time mapbench 16
  12300269260 cycles

with VM_CRANGE:

  $ time mapbench 1
  12969300915 cycles
  $ time mapbench 2
  15277712346 cycles
  $ time mapbench 16
  58582887683 cycles