ipv4: Cache dst in tunnels

Avoid doing a route lookup on every packet being tunneled.

In ip_tunnel.c cache the route returned from ip_route_output if
the tunnel is "connected" so that all the rouitng parameters are
taken from tunnel parms for a packet. Specifically, not NBMA tunnel
and tos is from tunnel parms (not inner packet).

Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>