[IPv6] route: FIB6 configuration using struct fib6_config

Replaces the struct in6_rtmsg based interface orignating from
the ioctl interface with a struct fib6_config based on. Allows
changing the interface without breaking the ioctl interface
and avoids passing on tons of parameters.

The recently introduced struct nl_info is used to pass on
netlink authorship information for notifications.

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