cifs: handle "guest" mount parameter

With the new mount API it can not handle empty strings for
mount parms ("guest" is mapped in userspace mount helper to
"user=") so we have to special case it as we do for the
password mount parm.

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