Over the years, the code and comments in vm_page_startup() have diverged in
one respect. When determining how many page structures to allocate,
contrary to what the comments say, the code does not account for the
overhead of a page structure per page of physical memory. This revision
changes the code to match the comments.

Reviewed by: kib, markj
MFC after: 6 weeks
Differential Revision: https://reviews.freebsd.org/D9081