kernel - SWAP CACHE part 13/many - More vm_pindex_t work for vm_objects on i386

kernel - SWAP CACHE part 13/many - More vm_pindex_t work for vm_objects on i386

* vm_object->size also needs to be a vm_pindex_t, e.g. when mmap()ing regular
  HAMMER files or block devices or HAMMER's own use of block devices,
  in order to support vm_object operations past the 16TB mark.

* Introduce a 64-bit-friendly trunc_page64() and round_page64(), just to
  make sure we don't cut off page alignment operations on 64-bit offsets.