selftest: af_unix: Make SCM_RIGHTS into OOB data.

scm_rights.c covers various test cases for inflight file descriptors
and garbage collector for AF_UNIX sockets.

Currently, SCM_RIGHTS messages are sent with 3-bytes string, and it's
not good for MSG_OOB cases, as SCM_RIGTS cmsg goes with the first 2-bytes,
which is non-OOB data.

Let's send SCM_RIGHTS messages with 1-byte character to pack SCM_RIGHTS
into OOB data.
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

tools/testing/selftests/net/af_unix/scm_rights.c

@@ -197,8 +197,8 @@ void __send_fd(struct __test_metadata *_metadata,
 	       const FIXTURE_VARIANT(scm_rights) *variant,
 	       int inflight, int receiver)
 {
-#define MSG "nop"
-#define MSGLEN 3
+#define MSG "x"
+#define MSGLEN 1
 	struct {
 		struct cmsghdr cmsghdr;
 		int fd[2];