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: 39b2e5cae4 ("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>

tools/testing/selftests/vm/madv_populate.c

@@ -20,6 +20,13 @@
 #include "../kselftest.h"
 #include "vm_util.h"
 
+#ifndef MADV_POPULATE_READ
+#define MADV_POPULATE_READ	22
+#endif /* MADV_POPULATE_READ */
+#ifndef MADV_POPULATE_WRITE
+#define MADV_POPULATE_WRITE	23
+#endif /* MADV_POPULATE_WRITE */
+
 /*
  * For now, we're using 2 MiB of private anonymous memory for all tests.
  */