macvlan: Don't propagate IFF_ALLMULTI changes on down interfaces.

Clearing the IFF_ALLMULTI flag on a down interface could cause an allmulti
overflow on the underlying interface.

Attempting the set IFF_ALLMULTI on the underlying interface would cause an
error and the log message:

"allmulti touches root, set allmulti failed."

Signed-off-by: Peter Christensen <pch@ordbogen.com>
Signed-off-by: David S. Miller <davem@davemloft.net>