ipv6: use xarray iterator to implement inet6_dump_ifinfo()

Prepare inet6_dump_ifinfo() to run with RCU protection
instead of RTNL and use for_each_netdev_dump() interface.

Also properly return 0 at the end of a dump, avoiding
an extra recvmsg() system call and RTNL acquisition.

Note that RTNL-less dumps need core changes, coming later
in the series.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>