selftests/bpf: Fix memory leak in msg_alloc_iov

In the msg_alloc_iov function, the iov pointer is only assigned to
msg->msg_iov after all memory allocations complete successfully.
Therefore, when a calloc failure triggers the unwind_iov cleanup branch,
we should use the local variable iov instead of msg->msg_iov.

Fixes: 753fb2ee09 ("bpf: sockmap, add msg_peek tests to test_sockmap")
Signed-off-by: Feng Yang <yangfeng@kylinos.cn>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20260707081434.539327-1-yangfeng59949@163.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

tools/testing/selftests/bpf/test_sockmap.c

@@ -435,7 +435,7 @@ static int msg_alloc_iov(struct msghdr *msg,
 	return 0;
 unwind_iov:
 	for (i--; i >= 0 ; i--)
-		free(msg->msg_iov[i].iov_base);
+		free(iov[i].iov_base);
 	free(iov);
 	return -ENOMEM;
 }