net: ipv6: remove nowait arg to rt6_fill_node

All callers of rt6_fill_node pass 0 for nowait arg. Remove the arg and
simplify rt6_fill_node accordingly.

rt6_fill_node passes the nowait of 0 to ip6mr_get_route. Remove the
nowait arg from it as well.

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