[IPV4]: Convert ipv4 route to use the new dst_entry 'next' pointer

This patch removes the rt_next pointer from 'struct rtable.u' union,
and renames u.rt_next to u.dst_rt_next.

It also moves 'struct flowi' right after 'struct dst_entry' to prepare
the gain on lookups.

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