sfxge(4): update multicast filter insertion algorithm

[arybchik]

Summary

When the multicast filters we're allowed to insert are controlled by the
hypervisor, it may be that we can insert some but not others. So we need to
have fallbacks where we insert any filters we can without rolling back
when one fails to insert.

Submitted by: Mark Spender <mspender at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week

Test Plan

Build tested, driver loaded, IPv6 ping checked.