kernel - Fix panic in zget() during heavy paging

kernel - Fix panic in zget() during heavy paging

* Fix a panic that can occur in zget() when a large number of swap info
  blocks are needed from the swap_zone.

  Fix by reformulation how new items are added to the zone.  This is a
  non-critical code path, but we make it a bit more SMP friendly anyway.

* Use the opportunity to get rid of the VM object ZONE_INTERRUPT uses.
  The two that are left are never destroyed, so there's no need to track
  the pages in an object.

Reported-by: tkusumi