SMB3: Warn user if trying to sign connection that authenticated as guest

commit c721c38957fb19982416f6be71aae7b30630d83b upstream.

It can be confusing if user ends up authenticated as guest but they
requested signing (server will return error validating signed packets)
so add log message for this.

Signed-off-by: Steve French <smfrench@gmail.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>