selftests/seccomp: Accept any valid fd in user_notification_addfd

This test expects fds to have specific values, which works fine
when the test is run standalone. However, the kselftest runner
consumes a couple of extra fds for redirection when running
tests, so the test fails when run via kselftest.

Change the test to pass on any valid fd number.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>