[DECNET]: Convert decnet route to use the new dst_entry 'next' pointer

This patch removes the next pointer from 'struct dn_route.u' union,
and renames u.rt_next to u.dst.dn_next.

It also moves 'struct flowi' right after 'struct dst_entry' to prepare
speedup lookups.

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