ipv6: speedup inet6_dump_addr()

When handling large number of netdevices, inet6_dump_addr()
is very slow because it has O(N^2) complexity.

Instead of scanning one single list, we can use the NETDEV_HASHENTRIES
sub lists of the dev_index hash table, and RCU lookups.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>