[PATCH 1/2] KVM: selftests: Fix MADV_COLLAPSE build failure on older toolchains

[Fuad Tabba <tabba@google.com>]

guest_memfd_test.c uses MADV_COLLAPSE unconditionally, but the
constant was only exposed in glibc 2.37. Distros still on older glibc
(e.g. Debian Bookworm, glibc 2.36) cannot build the test.

Add a compile-time fallback that defines MADV_COLLAPSE to its
kernel-ABI value when the libc header does not, matching the
established selftest pattern for sometimes-missing libc constants
(see selftests/bpf/uprobe_multi.c, selftests/mm/hugetlb-soft-offline.c).

Fixes: 9830209b4ae8 ("KVM: selftests: Test MADV_COLLAPSE on guest_memfd")
Signed-off-by: Fuad Tabba <tabba@google.com>
---
 tools/testing/selftests/kvm/guest_memfd_test.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c
index d6528c6f5e03..45990ae5f1e5 100644
--- a/tools/testing/selftests/kvm/guest_memfd_test.c
+++ b/tools/testing/selftests/kvm/guest_memfd_test.c
@@ -23,6 +23,10 @@
 #include "test_util.h"
 #include "ucall_common.h"
 
+#ifndef MADV_COLLAPSE
+#define MADV_COLLAPSE	25
+#endif
+
 static size_t page_size;
 
 static void test_file_read_write(int fd, size_t total_size)
-- 
2.54.0.563.g4f69b47b94-goog