[IPv6] iflink: Convert IPv6's RTM_GETLINK to use the new netlink api

By replacing the current method of exporting the device configuration
which included allocating a temporary buffer, copying ipv6_devconf
into it and copying that buffer into the message with a method that
uses nla_reserve() allowing to copy the device configuration directly
into the skb data buffer, a GFP_ATOMIC allocation could be removed.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>