selftests/nolibc: mmap_munmap_good: fix up return value

The other tests use 1 as failure, mmap_munmap_good uses -1 as failure,
let's fix up this.

Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>