cifs: try to handle the MUST SecurityFlags sanely

The cifs.ko SecurityFlags interface wins my award for worst-designed
interface ever, but we're sort of stuck with it since it's documented
and people do use it (even if it doesn't work correctly).

Case in point -- you can specify multiple sets of "MUST" flags. It makes
absolutely no sense, but you can do it.

What should the effect be in such a case? No one knows or seems to have
considered this so far, so let's define it now. If you try to specify
multiple MUST flags, clear any other MAY or MUST bits except for the
ones that involve signing.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Pavel Shilovsky <piastry@etersoft.ru>
Signed-off-by: Steve French <smfrench@gmail.com>