crypto/ssh: only show banner once

Only show the SSH banner once, even if the client attempts
authentication with the "none" type and resets the authentication
attempts.

Change-Id: I1a7aacb50abf00233ed4d06c60808aaf51a9d7be
Reviewed-on: https://go-review.googlesource.com/78544
Run-TryBot: Sam Whited <sam@samwhited.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>

ssh/server.go

@@ -316,6 +316,7 @@ func (s *connection) serverAuthenticate(config *ServerConfig) (*Permissions, err
 
 	authFailures := 0
 	var authErrs []error
+	var displayedBanner bool
 
 userAuthLoop:
 	for {
@@ -348,7 +349,8 @@ userAuthLoop:
 
 		s.user = userAuthReq.User
 
-		if authFailures == 0 && config.BannerCallback != nil {
+		if !displayedBanner && config.BannerCallback != nil {
+			displayedBanner = true
 			msg := config.BannerCallback(s)
 			if msg != "" {
 				bannerMsg := &userAuthBannerMsg{