MEDIUM: ring: protect the initialization of the initial reader offset

MEDIUM: ring: protect the initialization of the initial reader offset

Since we're going to remove the lock, there's no more way to prevent the
ring from being fed while we're attaching a client to it. We need to
freeze the buffer while looking at its head so that we can attach there
and have a trustable one. We could do it by setting the lock bit on the
tail offset but quite frankly we don't need to bother with that, attaching
a client is rare enough to permit a thread_isolate().