cifs: when ATTR_READONLY is set, only clear write bits on non-directories

On windows servers, ATTR_READONLY apparently either has no meaning or
serves as some sort of queue to certain applications for unrelated
behavior. This MS kbase article has details:

http://support.microsoft.com/kb/326549/

Don't clear the write bits directory mode when ATTR_READONLY is set.

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