posixshm tests: Fix occasional largepage_mprotect failures

largepage_mprotect maps a superpage and later extends the mapping. This
occasionally fails with ASLR disabled. To fix this, first try to
reserve a sufficiently large virtual address region.

Reported by: Jenkins
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 321e586e46115c69e7e833d592703d997a4ec477)