userfaultfd/selftests: don't rely on GNU extensions for random numbers

Patch series "Small userfaultfd selftest fixups", v2.

This patch (of 3):

Two arguments for doing this:

First, and maybe most importantly, the resulting code is significantly
shorter / simpler.

Then, we avoid using GNU libc extensions.  Why does this matter? It
makes testing userfaultfd with the selftest easier e.g.  on distros
which use something other than glibc (e.g., Alpine, which uses musl);
basically, it makes the test more portable.

Link: https://lkml.kernel.org/r/20210930212309.4001967-2-axelrasmussen@google.com


Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>