Merge branch 'bridge_batmanadv_exports'

Linus Lüssing says:

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bridge: multicast snooping exports #2

Some people pointed out to me that it might be helpful to add stubs for
the newly added multicast exports. That way e.g. batman-adv should continue
to be compile and useable without having to have a kernel compiled
with bridge code in the future. This is what the first patch is supposed
to do.

The second patch adds a third multicast export for the bridge which
e.g. batman-adv is supposed to use, too, soon: Just like the bridge
disables its multicast snooping activities if no querier is present,
batman-adv needs to do the same if bridges are involved.

These three exports should be the final ones needed to marry the bridge
multicast snooping with the batman-adv multicast optimizations recently
added for the 3.15 kernel, allowing to use these optimzations in common
setups having a bridge on top of e.g. bat0, too. So far these bridged
setups would fall back to simple flooding through the batman-adv mesh
network for any multicast packet entering bat0.

More information about the batman-adv multicast optimizations currently
implemented can be found here:

http://www.open-mesh.org/projects/batman-adv/wiki/Basic-multicast-optimizations

The integration on the batman-adv side could afterwards look like this,
for instance (now including the third export):

http://git.open-mesh.org/batman-adv.git/commitdiff/61e4f6af4b7a21ed4040f2e711d50c778e5b6d93?hp=6ae4281474675fbca5bedcf768972a32db586eb6
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