With multicast chaining, if e.g. a specific multicast filter is
inserted and the multicast mis-match filter is then inserted, both may
match a packet and cause it to be delivered.
Copy the behaviour of the Linux driver, which is to remove the old
filters first, on the basis that customers are more likely to be able
to handle drops than duplicates (see bug49178 comment 4).
Submitted by: Mark Spender <mspender at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week