selftests/mm: define MADV_PAGEOUT to fix compilation issues

If MADV_PAGEOUT is not defined (e.g., on AlmaLinux 8), compilation will
fail.  Let's fix that like khugepaged.c does by conditionally defining
MADV_PAGEOUT.

Link: https://lkml.kernel.org/r/20230109171255.488749-1-david@redhat.com
Fixes: 69c66add ("selftests/vm: anon_cow: test COW handling of anonymous memory")
Signed-off-by: David Hildenbrand <david@redhat.com>
Reported-by: Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/mm/cow.c

@@ -30,6 +30,9 @@
 #include "../kselftest.h"
 #include "vm_util.h"
 
+#ifndef MADV_PAGEOUT
+#define MADV_PAGEOUT 21
+#endif
 #ifndef MADV_COLLAPSE
 #define MADV_COLLAPSE 25
 #endif