ipv6: Report dead flag during route dump

Up until now the RTNH_F_DEAD flag was only reported in route dump when
the 'ignore_routes_with_linkdown' sysctl was set. This is expected as
dead routes were flushed otherwise.

The reliance on this sysctl is going to be removed, so we need to report
the flag regardless of the sysctl's value.

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