cifs: Allow LANMAN auth method for servers supporting unencapsulated authentication methods

This allows users to use LANMAN authentication on servers which support
unencapsulated authentication.

The patch fixes a regression where users using plaintext authentication
were no longer able to do so because of changed bought in by patch
3f618223

https://bugzilla.redhat.com/show_bug.cgi?id=1011621



Reported-by: Panos Kavalagios <Panagiotis.Kavalagios@eurodyn.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>