cifs: use CIFS_MAX_DOMAINNAME_LEN when converting the domain name

Currently in build_ntlmssp_auth_blob(), when converting the domain
name to UTF16, CIFS_MAX_USERNAME_LEN limit is used. It should be
CIFS_MAX_DOMAINNAME_LEN. This patch fixes this.

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>