kernel - Fix resident_count problems with kernel_pmap & related crashes

kernel - Fix resident_count problems with kernel_pmap & related crashes

* kernel_pmap.pm_stats.resident_count could get out of sync due to the
  lwbuf code calling kmem_free() on KVM with pages that were entered
  via pmap_kenter().  Delete the pages before calling kmem_free().

  This in turn caused various failures and kernel panics due to conditionals
  based on resident_count == 0.  Heavy sendfile() activity could trigger
  the bug but it was taking a few days for the resident_count to decrement
  to zero.

* Fix a similar issue in zdestroy() on ZONE_INTERRUPT zones.  Nothing calls
  this routine so it was not a vector for crashes.

* Document the case.