[IPV6]: Convert ipv6 route to use the new dst_entry 'next' pointer

This patch removes the next pointer from 'struct rt6_info.u' union,
and renames u.next to u.dst.rt6_next.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>