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.

Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D21115