kernel - Move action_list out of vm_page, and change flags from 16->32 bits

kernel - Move action_list out of vm_page, and change flags from 16->32 bits

* vm_page->action_list removed, saving 16 bytes per vm_page on x86_64
  (8 on i386).  Instead we add a flag to vm_page->flags, PG_ACTIONLIST,
  and store the action registrations in a global hash table.

* Change vm_page->flags from a 16 to 32 bits, plus pad, adding 4 bytes
  to the vm_page structure.

* x86_64 sizeof(struct vm_page) goes from 136 to 128, keeping in mind
  that it grew from 132 to 136 with the addition of ku_pagecnt.  So,
  net-net, we have still managed to decrease the size of struct vm_page
  by a little.