[IPV6]: Consolidate the ip6_pol_route_(input|output) pair

The difference in both functions is in the "id" passed to
the rt6_select, so just pass it as an extra argument from
two outer helpers.

This is minus 60 lines of code and 360 bytes of .text

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>