buffer: don't use ringbuffer in chunk

We can't use a ringbuffer on the chunk because it implies the
consumer would write to it to update the read position, which we
can't do because the chunk is read-only and might even be shared.
Go back to offset/size pairs, which can sortof do the same thing
if we want later when we keep a non-shared read pointer in the
consumer.
Keep alsa timestamp around and filled state for future.
mmap the input port meta/data/chunk as read-only.
Only do clock update requests when asked.

spa/tests/test-perf.c

@@ -167,7 +167,8 @@ init_buffer(struct data *data, struct spa_buffer **bufs, struct buffer *ba, int
 		b->datas[0].maxsize = size;
 		b->datas[0].data = malloc(size);
 		b->datas[0].chunk = &b->chunks[0];
-		spa_ringbuffer_set_avail(&b->datas[0].chunk->area, size);
+		b->datas[0].chunk->offset = 0;
+		b->datas[0].chunk->size = size;
 		b->datas[0].chunk->stride = 0;
 	}
 }