[CIFS] Fix mount failure when domain not specified

    Fixes Samba bugzilla #4176

    When users do not specify their domain on mount, 2.6.18 started sending
    default domain instead of a null domain (which was the only way on some
    servers to use a default domain).  Users of 2.6.18 who did not specify
    their domain name on mounts to certain common Windows servers that were
    members of a domain, but not the domain controller, would get mount
    failures which they did not get in 2.6.18

    This fixes that issue and should remove complaints about mount
    behavior changing.

Signed-off-by: Steve French <sfrench@us.ibm.com>