ssh: fail client auth immediately on receiving disconnect message

Fixes golang/go#66991

Change-Id: I60dd8a807578f162fda0e49bcd6fbf289d444396
GitHub-Last-Rev: f88329d35712873d0d7e3b39b9b11e7bfbc28e71
GitHub-Pull-Request: golang/crypto#293
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/581075
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Nicola Murino <nicola.murino@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

ssh/client_auth.go

@@ -71,6 +71,10 @@ func (c *connection) clientAuthenticate(config *ClientConfig) error {
 	for auth := AuthMethod(new(noneAuth)); auth != nil; {
 		ok, methods, err := auth.auth(sessionID, config.User, c.transport, config.Rand, extensions)
 		if err != nil {
+			// On disconnect, return error immediately
+			if _, ok := err.(*disconnectMsg); ok {
+				return err
+			}
 			// We return the error later if there is no other method left to
 			// try.
 			ok = authFailure