selftests/vm: madv_populate: fix missing MADV_POPULATE_(READ|WRITE) definitions

The tests fail to compile in some environments (e.g., Debian 11.5 on x86).
Let's simply conditionally define MADV_POPULATE_(READ|WRITE) if not
already defined, similar to how the khugepaged.c test handles it.

Link: https://lkml.kernel.org/r/20221205193716.276024-3-david@redhat.com


Fixes: 39b2e5ca ("selftests/vm: make MADV_POPULATE_(READ|WRITE) use in-tree headers")
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>