ipv6: Fix sporadic sendmsg -EINVAL when sending to multicast groups.
Thanks to excellent diagnosis by Eduard Guzovsky. The core problem is that on a network with lots of active multicast traffic, the neighbour cache can fill up. If we try to allocate a new route and thus neighbour cache entry, the bog-standard GC attempt the neighbour layer does in ineffective because route entries hold a reference to the existing neighbour entries and GC can only liberate entries with no references. IPV4 already has a way to handle this, by doing a route cache GC in such situations (when neigh attach returns -ENOBUFS). So simply mimick this on the ipv6 side. Tested-by: Eduard Guzovsky <firstname.lastname@example.org> Signed-off-by: David S. Miller <email@example.com>
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