MFC r305213,305319,305398

MFC r305213,305319,305398
  As an optimization to the machine-independent layer, change the machine-
  dependent pmap_ts_referenced() so that it updates the page's dirty field
  if a modified bit is found while counting reference bits.  This
  opportunistic update can be performed at low cost and can eliminate the
  need for some future calls to pmap_is_modified() by the machine-
  independent layer.

  Replace the number 4 in sparc64's pmap_ts_referenced() by
  PMAP_TS_REFERENCED_MAX, like we've done elsewhere, e.g., amd64.