vkernel64 - Fix pmap panic

* Fix a pmap panic complaining about the pm_active cpumask not being zero
  when the pmap is released/destroyed.

* The vkernel64 swtch code was improperly setting the active bits from
  the gd_other_cpus field instead of the gd_cpumask field, which wound
  up setting lots of bits instead of the one bit we wanted to set.

* Long-known bug was difficult to reproduce consistently enough to
  locate until Robert found a quick way.

Reproduced-by: Robert Garrett