Limit arg2 for mmap and mprotect to avoid writeable executable memory.

BUG=b:172212144
TEST=Cq passes

Change-Id: I5f969a506290d19224585b01c208c9603a3dc233
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/mosys/+/3942554
Commit-Queue: Allen Webb <allenwebb@google.com>
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>
Tested-by: Allen Webb <allenwebb@google.com>