Use the existing page size in libthr

Rather than calling getpagesize() twice use the value saved after the
first call to size a mmap allocation.

Reviewed by: kib, markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34983

(cherry picked from commit 86c500937c4514dda2319f193c4ccc043ffde2b3)