BUG/MINOR: mux-h2: count rejected DATA frames against the connection's flow control

BUG/MINOR: mux-h2: count rejected DATA frames against the connection's flow control

RFC9113 clarified a point regarding the payload from DATA frames sent to
closed streams. It must always be counted against the connection's flow
control. In practice it should really have no practical effect, but if
repeated upload attempts are aborted, this might cause the client's
window to progressively shrink since not being ACKed.

It's probably not necessary to backport this, unless another patch
depends on it.