net: ipv6: remove prefix arg to rt6_fill_node

The prefix arg to rt6_fill_node is non-0 in only 1 path - rt6_dump_route
where a user is requesting a prefix only dump. Simplify rt6_fill_node
by removing the prefix arg and moving the prefix check to rt6_dump_route.

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>