tst-mmap: fix two leaks

This patch fixes two places where tst-mmap.cc doesn't munmap() everything
it mmap()ed. This only makes a difference if doing leak detection on
tst-mmap.cc to see if our page-table handling code leaked memory.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>