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

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

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>
CC: Stable <stable@vger.kernel.org>