binderfs: add stress test for binderfs binder devices

This adds a stress test that should hopefully help us catch regressions
for [1], [2], and [3].

[1]: 2669b8b0 ("binder: prevent UAF for binderfs devices")
[2]: f0fe2c0f ("binder: prevent UAF for binderfs devices II")
[3]: 211b64e4

 ("binderfs: use refcount for binder control devices too")
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200313152420.138777-3-christian.brauner@ubuntu.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>