ksmbd: do not sign response to session request for guest login

If ksmbd.mountd is configured to assign unknown users to the guest account
("map to guest = bad user" in the config), ksmbd signs the response.

This is wrong according to MS-SMB2 3.3.5.5.3:
   12. If the SMB2_SESSION_FLAG_IS_GUEST bit is not set in the SessionFlags
   field, and Session.IsAnonymous is FALSE, the server MUST sign the
   final session setup response before sending it to the client, as
   follows:
    [...]

This fixes libsmb2 based applications failing to establish a session
("Wrong signature in received").

Fixes: e2f34481

 ("cifsd: add server-side procedures for SMB3")
Cc: stable@vger.kernel.org
Signed-off-by: Marios Makassikis <mmakassikis@freebox.fr>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>