ssh: return window credit for discarded extended data

handleData subtracts the length of every data packet from the receive
window, but extended data with a type code other than
SSH2_EXTENDED_DATA_STDERR is then silently discarded and, since no API
exists to read it, its window credit was never returned: adjustWindow
only runs as a side effect of reading a stream. A peer sending
extended data with an unknown type code could therefore permanently
shrink the channel window, down to a complete stall of the channel.

Credit the discarded bytes back through adjustWindow. adjustWindow
returns io.EOF if the local side has already sent a channel close;
ignore it like ReadExtended does, since an error returned here would
terminate the mux read loop and tear down the whole connection. For
comparison, OpenSSH rejects extended data with unexpected type codes
outright and never buffers it, so the dropped bytes are simply never
credited back to the misbehaving peer.

Fixes golang/go#80333

Change-Id: Ia311ce3ad008e7c89ce9752f1937c91374a4e74f
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/802901
Reviewed-by: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>