cifs: set domainName when a domain-key is used in multiuser

RHBZ: 1710429

When we use a domain-key to authenticate using multiuser we must also set
the domainnmame for the new volume as it will be used and passed to the server
in the NTLMSSP Domain-name.

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>