Multicast snooping fixes and suggestions

Hello everyone,

While testing the (very awesome!) bridge igmp/mld snooping support I came across
two issues which are breaking IPv6 multicast snooping and IPv6
non-link-local multicast on bridges with multicast snooping support enabled
in general. The first two patches shall fix these issues.

The third one addresses a potential bug on little endian machines which I noticed
during this little code reviewing. This patch is untested though, feedback welcome.

The fourth and fifth patch are a suggestion to also permit using the bridge multicast
snooping feature for link local multimedia multicast traffic. Therefore
using the transient multicast flag instead of the non-link-local scope criteria
seems to be a suitable solution at least for IPv6, in my opinion. Let me know what
you think about it.

Thanks for reviewing these patches.

Cheers,
Linus