cifs: allow sec=none mounts to work against servers that don't support extended security

In the case of sec=none, we're not sending a username or password, so
there's little benefit to mandating NTLMSSP auth. Allow it to use
unencapsulated auth in that case.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>