MINOR: mux-h2: Use the count value received from the SI in h2_rcv_buf()

MINOR: mux-h2: Use the count value received from the SI in h2_rcv_buf()

Now, the SI calls h2_rcv_buf() with the right count value. So we can rely on
it. Unlike the H1 multiplexer, it is fairly easier for the H2 multiplexer
because the HTX message already exists, we only transfer blocks from the H2S to
the channel. And this part is handled by htx_xfer_blks().