kernel: bridge, multicast-to-unicast: fix echoes on STA

kernel: bridge, multicast-to-unicast: fix echoes on STA

Currently, multicast packets from an STA are sent to any according
multicast listener directly through the bridge multicast-to-unicast
feature. Unfortunately, so far this includes the originating STA, too,
resulting in multicast packets being echo'ed back to the originating STA
if it itself is a multicast listener for that group.

This behaviour breaks IPv6 duplicate address detection: An IPv6 Neighbor
Solicitation for IPv6 Duplicate Address Detection is being echo'ed back,
resulting in the host falsely detecting an address collision, which
makes the node unable to claim an IPv6 address and use IPv6 in general.

Mac80211 unfortunately only prevents the echoes for us for multicast
frames. For the multicast frames cast to a unicast destination we'll
need to take care of excluding the originator ourselves.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Backport of r46765

git-svn-id: svn://svn.openwrt.org/openwrt/branches/chaos_calmer@47003 3c298f89-4303-0410-b956-a3cf2f4a3e73