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.