MFC r350463

MFC r350463
  In pmap_advise(), when we encounter a superpage mapping, we first demote
  the mapping and then destroy one of the 4 KB page mappings so that there
  is a potential trigger for repromotion.  Currently, we destroy the first
  4 KB page mapping that falls within the (current) superpage mapping or the
  virtual address range [sva, eva).  However, I have found empirically that
  destroying the last 4 KB mapping produces slightly better results,
  specifically, more promotions and fewer failed promotion attempts.
  Accordingly, this revision changes pmap_advise() to destroy the last 4 KB
  page mapping.  It also replaces some nearby uses of boolean_t with bool.