BUG/MINOR: mux-h2: Count padding for connection flow control on error path

BUG/MINOR: mux-h2: Count padding for connection flow control on error path

When DATA frame are received, we take care to update the counter used to
send WINDOW_UPDATE for the connection. It is also performed on error path
when DATA frames are processed. However, when this happened, only the frame
length was accounted while the padding must also be considered.

To fix the issue, the full frame length (h2c->dfl), which include the
padding length, must be added to the amount of newly received data
(h2c->rcvd_c).

The issue was introduced with commit eeacca75d ("BUG/MINOR: mux-h2: count
rejected DATA frames against the connection's flow control") and backported
to 2.8.

So this patch must be backported as far as 2.8.