sfxge(4): update multicast filter insertion algorithm

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
Differential Revision: https://reviews.freebsd.org/D6318