kernel: fix rtcache compilation with 4.18+ with IPv6 support

Please note that modified code isn't currently being compiled with
kernels 4.19+ due to the dropped CONFIG_NF_CONNTRACK_IPV6 in upstream
Linux. That requires a separated fix.

This fixes:
net/netfilter/nf_conntrack_rtcache.c: In function 'nf_rtcache_get_cookie':
net/netfilter/nf_conntrack_rtcache.c:82:11: error: 'const struct rt6_info' has no member named 'rt6i_node'; did you mean 'rt6i_idev'?
   if (rt->rt6i_node)
           ^~~~~~~~~
           rt6i_idev

IPv6 structs were reworked in upstream kernel by:
commit a64efe142f5e ("net/ipv6: introduce fib6_info struct and helpers")
commit 77634cc67dc1 ("net/ipv6: Remove unused code and variables for rt6_info")
commit 93c2fb253d17 ("net/ipv6: Rename fib6_info struct elements")

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>