ipv6: remove RTNL protection from inet6_dump_addr()

We can now remove RTNL acquisition while running
inet6_dump_addr(), inet6_dump_ifmcaddr()
and inet6_dump_ifacaddr().

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>