the other way that the swapslot for a page can change is if the page is
   modified.  if the page is paged out again, the swapslot is reassigned.


hmmm... isn't the code (uvm_pdaemon.c:uvmpd_scan_inactive()) saying that the
swap page is reallocated every time the page is swapped out, whether it was
modified or not?  ISTR the reason i did this is so we can do clustered swap
writes, rather than only being able to write NBPG bytes at a time.  but it
was far too long ago to recall precisely :-)