[PATCH bpf-next] selftests/bpf: fix uprobe_multi compilation error

[Alan Maguire <alan.maguire@xxxxxxxxxx>]

When building selftests, the following was seen:

uprobe_multi.c: In function ‘trigger_uprobe’:
uprobe_multi.c:108:40: error: ‘MADV_PAGEOUT’ undeclared (first use in this function)
  108 |                 madvise(addr, page_sz, MADV_PAGEOUT);
      |                                        ^~~~~~~~~~~~
uprobe_multi.c:108:40: note: each undeclared identifier is reported only once for each function it appears in
make: *** [Makefile:850: bpf-next/tools/testing/selftests/bpf/uprobe_multi] Error 1

...even with updated UAPI headers. It seems the above value is
defined in UAPI <linux/mman.h> but including that file triggers
other redefinition errors.  Simplest solution is to add a
guarded definition, as was done for MADV_POPULATE_READ.

Fixes: 3c217a182018 ("selftests/bpf: add build ID tests")
Signed-off-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
---
 tools/testing/selftests/bpf/uprobe_multi.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tools/testing/selftests/bpf/uprobe_multi.c b/tools/testing/selftests/bpf/uprobe_multi.c
index c7828b13e5ff..dd38dc68f635 100644
--- a/tools/testing/selftests/bpf/uprobe_multi.c
+++ b/tools/testing/selftests/bpf/uprobe_multi.c
@@ -12,6 +12,10 @@
 #define MADV_POPULATE_READ 22
 #endif
 
+#ifndef MADV_PAGEOUT
+#define MADV_PAGEOUT 21
+#endif
+
 int __attribute__((weak)) uprobe(void)
 {
 	return 0;
-- 
2.43.5