ipv6: Refactor fib6_ignore_linkdown

fib6_ignore_linkdown takes a fib6_info but only looks at the net_device
and its IPv6 config. Change it to take a net_device over a fib6_info as
its input argument.

In addition, move it to a header file to make the check inline and usable
later with IPv4 code without going through the ipv6 stub, and rename to
ip6_ignore_linkdown since it is only checking the setting based on the
ipv6 struct on a device.

Signed-off-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>